They knew they could get paid by posting in the other forum and knew mixers would be paying the highest rates therefore they followed where another revenue stream went. I have not read any reason from members stating they went there (or want to go there) in order to improve the other forum with their contributions.
Well, that's the sad truth. I refuse to believe someone makes an account there for other reason than money. However, I don't believe the overwhelming majority in bitcointalk would voluntarily contribute to their fellow forum users either. Perhaps occasionally, but certainly not with the same eagerness and frequency.
I can understand criticism about payments and weird rules there, but let's not forget about evil IP that is not available in any other forum, asking people to pay Bitcoin to complete registration is only available on Bitcointalk forum.
Isn't there a list of users who can whitelist you for free?
Another thing about funds for servers and money for maintaining website, it's easy to criticize others when bitcointalk collected enough money from donations and ads to last a lifetime
That is precisely why no other forum can compete. When you have enough money to operate for years ahead, without ads, with paid moderation (even if not enough), even with a YouTube series, you can
at least be the one with the aim to be as free as possible.