98% of Iranians are Muslims and Shah was fighting Islam. for example ban of Hijab and many other anti Islam policies. so many Islamic scholars were opposing him for a long time and Khomeini became the leader this anti Shah movement. in fact all this movement was based on Islam. and this is why it is called Islamic revolution. also after Revolution all the laws of country was revised based on Islamic laws.

As a young foreign woman married to a Iranian in 1979 I agree with the statement below. I never wore Hijab anywhere in Iran till the Ayatollah' demanded we foreign woman who were NOT raised as Muslims had to comply. Our freedoms were restricted not by choice and even when we supported the revolution at that time every Iranian no matter what he believed supported and fought for it and died. We did this not to exchange one dictator for another. We did it for the right to choose and many many families are still displaced today>.

What? Shah has never banned hijab in Iran and usually was protecting of Ayatollah's. all women were free to choose their own covers. they could have hijab or not in public places even at university. Please do not distort the history specially contemporary history. I never lied Shah but can you honestly say what Iran has now is better. I doubt it My son is half Iranian and bright its a shame he cannot share his talents for Iran. Sadly I doubt if he or many other Iranians with foreign mothers will ever see Iran. I am not Iranian I am Scottish but since I am married to an Iranian I claim the right to speak for an IRAN I helped fight for .It pains me to know all the young Iranians who died for nothing. I saw Evin prison right after the revolution I was there the Shah did not do right by his people history tells us that if not from him we would have no Ayatollah'.

Because this question has been edited many times I have to clarify that I am answering the version that asks:

What caused the Iranian 1979 revolution to become Islamic?

Short Answer (more suited for causal conversations in bars): It was easier to portray the Shah as anti Islamic ruler in league with the Western powers bent on destroying Islam in an Islamic majority country.

Long Answer: This explains why it was easier to do so.

Background

N.B.:- I mainly quote from two type of sources --a book and online sources. Citations numbered 1 refer to the book "The Great War for Civilization" by Robert Fisk. The specific chapter I draw my conclusions from is Chapter 4, The Carpet Weavers.

All other sources will be clearly mentioned.

To understand the Islamic Revolution of 1979 we should put the Shah's rule into perspective.

Back in 1953 Iran was under the rule of the Pahlavi dynasty and Mohammad Reza Shah Pahlavi was the king. In that year, Iran's only^1 democratically elected Prime Minister, Mohammad Mosaddegh, aided by a unanimous parliamentary vote, successfully nationalised the Iranian Petroleum Industry^4. As a result the Anglo-Iranian Oil Company (AIOC, later British Petroleum) lost all of its oil refineries in that country. The AIOC was at that time the chief source of revenue to Her Majesty's Government in the UK. Mossadeq also tended to lean towards the USSR^1.

The former irked the British administration and the latter US administration. Together, these two countries, under the administration of Churchill and Eisenhower respectively, designed the Operation Ajax and successfully deposed Mossadeq in a coup d'état^4,^1. The two administrations jointly selected Fazlollah Zahedi^4,^1 to become the next Prime Minister of Iran.

Following the deposition of Mossadeq, the Shah worked closely with the CIA^8 and introduced pro British/American policies in Iran. The Shah was also keen on pushing through what was called the White Revolution. The White Revolution was initiated by a popular referendum. It started with intense land reforms that abolished feudalism in Iran. Many government owned enterprises were privatised. Women got the right to vote. Free and compulsory (western) education was introduced. Workers got the right to own shares in their factories. There were several campaigns against “Unreasonable profiteering” and many wealthy businessmen were heavily fined or imprisoned. The Shah also established anti corruption bureaus.

While many of these moves were supposed to close Iran’s wealth gap, give more civil rights to its citizens, and clean up the government they came under intense criticism from the [mainly Islamic] clergy who opposed the women’s rights movements, western education, induction of non Muslim members to the Parliaments, and minority voting. The reforms also attracted criticism from the landlords and business owners who opposed the land reforms, industrial reforms, price control measures, and workers’ rights movements. And last, as the Shah cemented his authority over the administration —limited it to a single political party (the Rastakhiz) and his core loyal group —new groups of opponents who desired political power were created.

It is also important to bear in mind that the Shah and his dynasty associated themselves with the millennia old Persian monarchy and not the Islamic monarchy. For example, the Shah changed the calendar to start year 1 not with Prophet Mohammad’s pilgrimage to Medina but with the birth of Cyrus the Great. In 1971, as another example, he spent more than US $100 million^1 to celebrate the 2500 years of Iranian monarchy.

The Shah also used to frequently call the White Revolution a process for “Westernization”. While the term White officially referred to the revolution being bloodless and not racial characteristics of the Western people and Westernization officially meant “development and liberalization”, these terms nonetheless associated him with the CIA and USA even more closely.

The Revolution

N.B.:- I summarise Fisk’s observations he made while in Iran and a well cited Wikipedia article now.

With enough powerful groups in opposition by 1970 the Shah could only hope for people’s support —the very [supposed] benefactors of his reforms. Let us now look at the Iranian demographics —an overwhelming Islamic majority and a predominately young educated but unemployed crowd^10. As with all revolutions, however we like to believe that it was all planned, much of it actually is shaped along the course.

The Iranian revolution started with protests against the one-party system, unemployment, and corruption in the government 10 years before the actual revolution (the specific protests against the one party system obviously began after 1975 when the party was founded). By that time, Khomeini had had opposed the Shah for a long time and had been, as a result, exiled. He had hidden himself in France. He was a charismatic figure in Iran^1 and the clergy portrayed him as following the footsteps of Hussaeyn Ibn Ali, a Shia Imam, who had opposed a tyrant. Seeing his growing popularity, many sections of clergy back at home attempted to galavinze the crowds by presenting Islamic causes —some of these were not in league with Khomeini. Islamic plans for modernization (most notably by Ali Shariati and Abulhassan Bani-sadar) found acceptance in the general populace. There were Marxist sections too but they didn’t strike the resonance that the Islamic movement did as the Shah’s policies weren’t very different from what the Marxists proposed (one party system, political restrictions).

But the Shah’s policies were very different from what the proponents of various Islamic movements proposed. The Shah’s “anti Islamic” reform moves were heavily advertised in pamphlets and speeches^1. His pro CIA stance, his previous collaboration in Operation Ajax, his pro Israel stance, and a general move towards Westernization were demonized.^1. The Islamic movement promised to protect Iran from foreigners (who were blamed for all the problems) and bring the Iranian law in line with the Sharia. Given the target approach, the crowd’s support followed.

That is why the revolution became Islamic in nature.