I don't think it has much to do with financial gain; I think it's mainly due to feedees gradually coming out of the closet and posting pics of themselves. There was a slow chain reaction--the more who came out of the closet and posted pics, the more who were inspired to do the same, until now there are quite a lot of them.
It was also helped by fantasyfeeder and curvage. These are much better than their primary predecessors, Dimensions and the yahoo groups. Dimensions has never been completely comfortable with feeders and feedees (even the "Weight Room" had to be split up into three subgroups!) And yahoo groups keep going stagnant and spam-choked; you always have to find new ones, and most of them are awful!
Fantasy Feeder an unabashed feeder site, didn't have to worry so much about offending anti-feederites--why come to ff in the first place if you're anti-feeder? And that "Fantasy" in the title made it more comfortable for people just getting their feet wet.
Anyway, more and more people have been out about this (some faceless, some not) and in the last year it's really snowballed.
That's my take on it anyway.
16 years