If the paysite has killer content that can keep members rebilling, then they want to do PPS.. because a $40 PPS is cheaper than paying 50% of a 19.95/month for a year.
Affiliates want instant gratification for their traffic, so they go for the PPS.
Those that do rev-share might fall into one of the following reasons:
1) don't have the float to carry having a big hit of signups and being able to survive for 2-3 months until the rebills are profitable
2) aren't sure of their content, and rev-share is the best way to not be in the black on the signup
3) have great retaining content but like to share the wealth with their affiliates
4) (feel free to suggest some other reasons why programs have rev-share).
PPS also invites shaving alot more than rev-share....
Those companies that have the capital to sustain operations and do PPS, are the ones that attract the big traffic and making the big dollars with their long-term plans.
Fight the necessary evil!