It's all about one's comfort level.

I don't like playing on a site with too few buyins. For example, if I have only $4000 on PS, I am not going to sit at 4+ tables of $400NL, even if I do have $40, 000 in my bank.
Once I withdraw money, it's never going to get redeposited again.

That's just the way I feel, and I am sure there are a lot of people that feel the same.