Why can't people change their denomination?

Someone who knows that the doctrines of his present church are true and revealed by God would commit a sin if he were to change to a church with different doctrines, which he knows to be false.

A person embrace a denomination usually based on two things:

1. They grew up in the denomination
2. They are discipled by people from a certain denomination

And neither of these are wrong, because denominations help us to create a frame of mind on how to relate to God when we started as a young Christian. But here is the crux of the problem, many Christian made the denomination their source of power and knowledge for spiritual growth, instead of The Holy Spirit (ref. 1 Corinthians 2:10-16).

This is why we see people debate endlessly about denomination-specific doctrines. Instead of letting ourselves be grown by God (ref. 1 Corinthians 3:6-7), we try to grow ourselves by choosing to adopt the beliefs of the denomination founders or teachers.

The Apostle Paul already warned that we should not choose any (human) sides when it comes to our growth. We should have the mind of Christ, and by the guidance of the Holy Spirit, our final goal is to be Christ's image ('icon' in Greek), not becoming the paragon or champion of the denomination that we belong to.

I do not belong to any denominations, but I learned great truths by studying those saints whose heart is solely consumed and focused towards God. I learned from Saint Thomas Aquinas (Catholic), I learned from Alistair Begg (Evangelical Reformed), R.C. Sproul (Presbytarian), I learned from Eli Stanley Jones (Methodist), and many, many more.

How do I reconcile the points where they differ? I don't, I wait for the Holy Spirit to direct me towards the ones that will grow me closer to God. And no, none of the denominations has all of the truths, but almost all of the denomination teachings can be used for our growth by God. Not by ourselves nor by any other human beings.

So, can we change denominations? By all means, but don't focus on the denominations, focus on Christ, read and study the Scriptures with a submitting, and waiting heart, and God through His own Holy Spirit will grow you from the inside. It is a marvelous experience.

In general people change their denominations all the time. This is especially true between different Protestant denominations. I've been Anglican, Pentecostal, Mennonite and United Church.

In the specific case you quote, the Jehovah's Witnesses quite literally believe that all other Branches of Christianity are apostate - i.e. they are not really Christians at all. Most denominations are nowhere near that exclusive. The Witnesses are a Restorationist church, who believe that the entire church went off the rails and started worshipping a false God (the Trinity), and that their original leader had a specific revelation that told him to restart the true church. To them, marrying a Catholic is no different from marrying a non-Christian.

Why are people reluctant to change denomination? Many aren't, but where they are it is usually because they believe what their denomination teaches and don't want to be part of a church that teaches something different. The amount of reluctance depends on how important that person perceives the differences. There is also the question of leving an established community. For people changing denomination may be perceived as a slight to family or traditions. Changing church is a significant upheaval, and people rarely change without a good reason.

Hindrances to changing denomination. Some denominations require rebaptism, or some other initiation before accepting a new member. That's rarely onerous, but it can be a hindrance for some. In a free country there is pretty much no reason why people cannot change denomination if they really want.

For people who do change denomination, there will be a time of building of a new community to replace the one left. There may be a learning of new ways of doing things. The difficulty of this depends on the magnitude of the transition.

The primary reason why serious Christians cannot switch denominations is because the doctrines surrounding salvation have eternal consequences.

For instance, the Catholic Church clearly teaches that the Eucharist is the source and summit of the Christian faith, and that without a valid transubstantial reception of the Eucharist the human soul will eventually wither away.

On the other hand, most Baptist camps teach and believe that the Eucharist is only a symbolic memorial of the Last Supper, and really has nothing to do with salvific Grace other than a Christ-given ordinance of worship.

...two totally different denominational doctrines...

(I chose these two denominations for no specific anti-ecclesiastical reason other than comparison of doctrine)

Another example is justification by Grace alone through a working faith taught by pre-Reformation churches versus justification by faith alone taught by the majority of Protestant denominations.

On crucially important doctrines such as these, you are either right or wrong. This is why its infinitely and eternally important to find the right denomination (well...at least as close as possible).

Of course there are many other superfulous reasons why people stay where they are, such as familial pressure and tradition.

I really hate focusing on differences rather than what we all have in common, but this question seems to draw lines in the sand.

It must be said that there will always be overlapping areas of grey, and the Holy Spirit knows what he is doing.