Why are Catholics singled out from "Christians"?

You have to look in the history to understand what happened and how Catholic Church emerged. Before 1024, there were no Catholics or Orthodox people, just Christians. Church, in Latin, basilica = the total teaches of Christ. Catholic = universal, so, it's Called the universal Church. But we have to take care of this term, because they made a universal accepted rule, ignoring most of the Apostolic Canons, which the Orthodox Church did keep, this is where the name of the Orthodox Church come from: Orthodox = right belief, the right law. Every time someone wanted to change the law, for their own use, or to make the rules more easily accepted, in order to manipulate people, a new religion emerged. Look also a the Greek Catholic church history. If you look over, the grates empires did try to conquer many peoples and convert them to the Catholic Church because this way they would obey much easily. Also, the Catholic Church tried to conquer the Holy Land, which did not wanted to obey to their new rules. Since then more and more people started to feel unsatisfied by their teachings, but were disoriented, so they create more and more religions, without having contact with the old, unchanged and untouched Law of Christ.

You can search for the Rudder (Pedalion), to understand the old Law, also, you ca look for: 1. the Great Schism history. 2. Knights Templar https://orthodoxwiki.org/The_Rudder

The canons of the Apostles in the Rudder are older than any law that emerged and changed the true Christian law. This is why anyone who does't obey to these teaches is not considered true christian. Here are some canons extracted from the Rudder:

46. We order any Bishop, or Presbyter, that has accepted any heretics’ Baptism, or sacrifice, to be deposed; for "what consonancy hath Christ with Beliart or what part hath the believer with an infidel?"

47. If a Bishop or Presbyter baptize anew anyone that has had a true baptism, of fail to baptize anyone that has been polluted by the impious, let him be deposed, on the ground that he is mocking the Cross and death of the Lord and railing to distinguish priests from pseudopriests.

http://www.holytrinitymission.org/books/english/cannons_apostles_rudder.htm

Note: due to the ecumenism, the Universal Church has grown, no matter the beliefs and the laws that a church has. Most of the churches that called themselves Christians, must obey to some imposed rules. This is why all the world's official churches are heretic, and not obeying to the Law. I found very small communities that succeeded to keep the true Law, but with great sacrifice and torture resistance. They never forced anyone to be like them, never shed blood to make someone be like them, but they always were the ones killed and tortured. Also, they are not recognized as religion because they do not want to embrace the ecumenism and the priests do live for peoples mercy.

The reason is simply one of semantics.

RELIGION is "a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."

Wiki takes a different view, suggesting it is only a recent definition, though yet providing basically the same delineations. "There is no scholarly consensus over what precisely constitutes a religion.1 It may be defined as a cultural system of designated behaviors and practices, world views, texts, sanctified places, prophesies, ethics, or organizations, that relate humanity to the supernatural, transcendental, or spiritual."

So based on those definitions, it's not that Catholics are singled out, but rather because of material doctrinal differences between Catholics, Orthodox, and Protestant, each actually may in fact be considered a different religion from each other, just as other religions are different also.

Each participant thereof (Protestant, Orthodox, Catholic) may of course claim to be THE Christian religion, but of course, that is a different subject. What defines a Christian, rather than what defines a religion?

50% of Christians are Catholic and the majority of the others are Protestant. They are very similar, but the differences are striking. One supersedes the Bible and focuses on ritual and the other is focused within the Bible. There are also differences in regards to praying to saints and others. Finally, there is the whole transubstantiation thing - does the bread BECOME Christ or is it symbolic only. The differences almost make the two groups different religions - in that they cannot be reconciled. For example, I cannot see Protestants conforming to the Pope's authority as the vicar of Christ and I cannot see Catholics giving up the Pope.

Some people emphasize these distinctions; but that's not very catholic is it?

Below this is rhetorical and thought provoking in nature:

At what point do we distinguish between worshiping the same God differently and worshiping a different or false God? I'm stretching here, but what is the difference between a monotheistic religion and a false religion? If you believe that it all comes down to believing in the God of Abraham then you have to believe that Judaism and Islam also believe in the same God but worship him differently - an unpopular belief for Christians but not uncommon of Judaism and Islam (see this and Ahl al-Kitāb)

There are many Protestant groups that regard the Roman Catholic Church as a non-Christian cult. There are many others that merely consider it to be a Christian group with teachings that they disagree with. Many years ago the Christian Research Institute (CRI) conducted a research project to come up with a good definition of what a cult is and whether the Catholic Church met that definition. They decided that it did not. Here is one such paper on their website:

http://www.equip.org/article/is-catholicism-a-cult/

One part of their assessment was that most cults tend to deny some important teaching of Christianity, and the Roman Catholic Church denies none of them. The only problem that CRI found was where the church added teachings not supported in scripture.

Just to add a point neglected in the other answers.

Though Catholics clearly acknowledge that they are Christian (i.e., believing that Jesus is God), some Catholics might exclude themselves from being grouped with "Christianity" in a general sense due more to social reasoning vs. strictly theological purposes. Of course Catholics are Christians, but in a social sense, many Catholics do not want to be associated with the word "Christianity" because of the negative connotation that the word "Christianity" has developed, especially in the United States. As a result, many Catholics, especially in the United States, will often separate themselves from other Christian sects in order to avoid gaining a shared reputation of being illogical or backwards.

Many Catholics who study the likes of Thomas Aquinas, Augustine, Irenaeus and other apologists have developed their faith using a very scholastic approach guided by logical reasoning. The long history and list of storied apologists that have come and gone in the Catholic church provide a rich history to support their beliefs. Further, almost all modern science and education have their roots in the Catholic faith and their scholasticism. I would posit that many of these Catholics believe that their faith, then, is reasonable, defensible, and consistent. They likely try to "separate" themselves from other sects of Christianity to avoid "tarnishing" their faith's reputation and to avoid appearing less reasonable, objective or consistent.

In contrast to Catholicism, many (not all) fundamentalist sects of Christianity are considered to be illogical, backwards, and confused, especially in the United States. This is due primarily to their positions on science (e.g., age of world, evolution, etc.) as well as inconsistent approaches to their own fundamental beliefs (e.g., they are "pro-life" but only support protecting the lives of unborn fetuses and often not those of poor persons, prisoners or that of the environment). To be honest, I would suggest that many of the Christian sects in the U.S. have traded true faith and religion for the blinded following of a conservative political party or heretical false prophets. Some of these sects of Christianity are created solely upon heresies or, worse, on the greedy motives of "church" founders -- look no further than the number of TV and radio pastors that take people's money to buy lavish gifts for themselves in exchange for the saving and healing of donor's souls. (Interestingly, these confused sects are returning to the practices that Luther so fervently opposed 500 years ago!).

Overall, these issues have created a bit of a "bad reputation" for Christianity in the United States. The issue, of course, is not all of Christianity, but nevertheless, the word "Christian" in the U.S. often is associated with backwards thinking and a strict political ideology because of the beliefs and practices of many of the 100,000s sects that exist.

Catholics don't believe that the world's history is confined by the text of the Bible like numerous fundamentalist sects of Christianity. The Catholic Catechism as well as other texts (most recently Pope Francis's Laudato Si' and other sermons) also demonstrate that the Catholic church is (perhaps not in practice but in belief) very consistent in it's faith (e.g., "pro-life" means pro everyone's and everything's life). The Catholic church is not perfect, but many of it's Church believe that the church's dogma and catechism are nevertheless reasonable. Though many people despise Catholicism, it's often for the Church's rigidity, "priest problem" or unscrupulous history, not for a failure in their reasoning.

My best interpretation of is because of some of the core ways that beliefs are formed and held. The comparison "Catholic vs. Christian" isn't the best because they both are technically a form of Christianity. A better version is "Catholic vs. Protestant". While it is true that they both believe in the same God and the same Bible, Catholics tend to believe that the pope is the "closest person on earth to God" so-to-speak. Therefore, the pope is usually the best one to interpret the Bible. However, Protestantism tends to lean more toward the idea that any person can interpret the Bible how they wish (hence why there are so many protestant sects and only 1 catholic sect).

There are also other types of differences, for example, Catholics believe that saints can "deliver" your prayer to God or Jesus. The most common saint is the Virgin Mary (who is held in high regard b/c she is the mother of the savior of mankind), and even have a dedicated prayer for her (the "Hail Mary"). However, Protestants believe that you should only pray to God or Jesus. Other differences include that Catholics believe in a more ritualistic form of worship (memorized prayers, same routine every Sunday/every year, etc.) while protestants are more open. Also, the method of salvation is also a little different. Fundamentally, you can only get to Heaven through Jesus Christ, but Catholics also build upon that idea a little bit more with baptism, holy matrimony, etc.

Long story short, it is mostly the smaller details of what they believe, but the big concepts they share.

Source: Was raised a Catholic but converted to Protestant.

There are a couple main reasons, some of them I believe subconscious, for why Catholics are singled out.

1. They believe themselves to be exclusively the Church that Christ established, and other Christian sects (with a few exceptions) as adhering to heresies which bar them from ecclesiastical unity with them. This is an example of the Catholic Church drawing a distinction between themselves and other Christian sects.
2. The "Protestants" are called Protestants because they formed out of "protest" against perceived corruption in the policies, praxis, and theology of the Roman Catholic Church. This is an example of the Protestant sects drawing a distinction between themselves and the Catholics, although most still would graciously assert that, as Catholics believe Jesus is God, Catholics are saved despite their theology and praxis.

Another point:

Every main branch of Christianity is in some sense "singled out". It may simply seem that the Catholics are "singled out" because their branch of Christianity is united as a single sect under the Pope, while the Protestants are 40,000+ (or something) fragmented sects of Christianity existing in large numbers in most of the countries where Catholicism has a big presence, but are united only in in the confession that Jesus is God (a union through the least common denominator, choosing to name all other doctrines as secondary in importance when it comes to unity and, in most cases, also salvation).

There are also, however, other branches of Christianity which are also separate, or "singled out".

For instance, the Eastern Orthodox (Calcedonian) Communion, the Oriental Orthodox (non-Calcedonian ) Communion, the Assyrian Church of the East, and the modern-day Arians (Jehova's witnesses, Unitarians, Mormons, etc.).

It's just what happens when there is a major departure in theological understanding in a religion - those who believe one way need some way of differentiating themselves from those that believe another way that is deemed improper or incorrect by some adherents of the same religious umbrella (in this case, Christianity).

At the end of the day, the Catholics single themselves out as the only true Christians because they believe any non - Catholic sects to be in heresy, and therefore, not a 100% true expression of Christianity.