You asked, this is my opinon:
I say again: money (mostly). Vanilla realms require the cheapest hardware to play on them. They are the most simple version of WoW in all terms, you can literally play it on a toaster. Think about it how many ppl use that cheap hardware. There are so many of them, you can't even imagine, and you can't expect them to even pay a dime for anything on the Internet.
There is an other portion however: trend. Millennials (I use this term again, because that age group started this crusade, not the ones who actually played vanilla) tend to pick up the stupidest trends and they do it and fight for it in the name of popularity, like their life would depend on it. This is a much deeper social aspect, but you have to understand what social media brought to the life of these youngsters. It's the compliance constraint they face, not just at school or at home, but on the Internet too. They feel the urge to stay in the curve, keep up their popularity, be cool, belong to a specific community. If they can't keep it up they become seriously depressed, burned out that can even lead to suicide in extreme cases.
That's why we are at more than 66 pages of this thread, and this is only one of many. These youngsters feel that they must fight for their "trend" in order to keep up the "coolness", despite they have no idea how life works, how a private company works, they don't accept the answer "no". This is also because of an other social aspect, making it simple: they get everything on a silver plate. They can cry and whine at home for mommy and even mommy can't say "no". They get used to it, completely washing away the limits and borders of real life and family life and they think this is normal. It's not, just no one taught it to them what is life about: compromises, priorities and resposibility.
This is my answer for it. They make this trend their crusade, they even end up on private servers, just to protect their pride on any way.
(p.s.: Dear moderator, I'm not intended to talk about private servers, but it can't be untouched, because it comes up all the time regarding to vanilla or other servers where the youngsters above collect their experience and "fuel" for this thread. Thank you.)