My uneducated guess is that it is because Christianity, Islam, Sikhism, Buddhism, and Judaism were all backed by governments and military forces during the initial stages of expansion. I don't believe there are any large religions for which this is not true - Hinduism is too old for us to say much about its origins, but there was a time when Buddhism was becoming extremely popular, and power was involved in re-establishing Hinduism.
If I'm right, then the thing that causes small memeplexes to become big memeplexes is the successful conversion of a few powerful and influential people (and that process happens through random drift in the case of religion)
Also, I think Christianity, Islam, and Judaism are the only religions which care about whether or not you believe them. (As in, members think that belief itself has consequences and so they aught to care what others believe). It's harder to leave these religions, with shadows of hell hanging over you. I think that in most other religions, people can sort of vaguely redirect worship from one set of symbols to another without really rejecting old beliefs and accepting new ones in a way that is consistent with "brainwashing" - it's more or less immaterial which religion they are following. I've got relatives who pray to little pictures of Jesus along with other Hindu idols, and I don't think they realize how odd this would seem to a Christian. The notion that deviation from a religious orthodoxy is bad tends to be absent, and I imagine that this makes conversion easier.
Some old SIAI work of mine. Researching this was very difficult because the relevant religious studies area, while apparently completely repudiating most public beliefs about the subject (eg. the effectiveness of brainwashing, how damaging cults are, how large they are, whether that’s even a meaningful category which can be distinguished from mainstream religions rather than a hidden inference - a claim, I will note, which is much more plausible when you consider how abusive Scientology is to its members as compared to how abusive the Catholic Church has been etc), prefer to publish their research in book form, which makes it very hard to review any of it. Some of the key citation were papers - but the cult panic was so long ago that most of them are not online or have been digitized! I recently added some cites and realized I had not touched the draft in a year; so while this collection of notes is not really up to my preferred standards, I’m simply posting it for what it’s worth. (One lesson to take away from this is that controlling uploaded human brains will not be nearly as simple & easy as applying classic ‘brainwashing’ strategies - because those don’t actually work.)
Reading through the literature and especially the law review articles (courts flirted disconcertingly much with licensing kidnapping and abandoning free speech), I was reminded very heavily - and not in a good way - of the War on Terror.
Old American POW studies:
Started the myth of effective brain-washing. But in practice, cult attrition rates are very high! (As makes sense: if cults did not have high attrition rates, they would long ago have dominated the world due to exponential growth.) This attrition claim is made all over the literature, with some example citations being:
a back of the envelope estimate for Scientology by Steve Plakos in 2000:
Iannaccone 2003, “The Market for Martyrs” (quasi-review)
Singer in particular has been heavily criticized; “Cult/Brainwashing Cases and Freedom of Religion”, Richardson 1991:
“Overcoming The Bondage Of Victimization: A Critical Evaluation of Cult Mind Control Theories”, Bob and Gretchen Passantino Cornerstone Magazine 1994:
Gomes, Unmasking the Cults (Wikipedia quote):
“Psychological Manipulation and Society”, book review of Spying in Guruland: Inside Britain’s Cults, Shaw 1994
Anthony & Robbins 1992, “Law, Social Science and the ‘Brainwashing’ Exception to the First Amendment”:
“Brainwashed! Scholars of cults accuse each other of bad faith”, by Charlotte Allen, Lingua Franca Dec/Jan 1998: