How about the variety of posts. DarkStar_ for example has in some cases specifically requested applicants from certain boards apparently because Chipmixer wants more exposure in those boards. On the flip side, I don't think they want to spend too much of their budget on one user who posts e.g. entirely in Gambling.

And don't start with the bullshit of a great poster being able to post all over the forum LOL. Or being able to make 100+ posts per week without negative effects on quality.

In the end it's a business decision despite some account farmers suggesting that it might have something to do with politics. If some entity, like Yobit recently, thinks it's ok to have a massive campaign with a 20-post daily limit, they'll do it. And then they'll reduce it to 5 posts per day, I wonder why.