They use paypal because its so easy, they forced to use the high fees gateway because they where so horny for the money it never occurred to them they would get screwed (and not in the good way) by paypal.
webmoney is no option since its completely difficult to use for none Russians (like googlebuster mentioned), payza(alertpay) and Libertyreserve are for scammers even though payza tried to go legit, their support is no where to be seen, and they even started to request extreme high fees to filehosts, bitcoins is for criminals (lots of shops on tor where you can buy guns and drugs with bitcoins).
Basically they where so sure of their money making they never searched for a good alternative.