mostly because the social stigma. it is not an acceptable job to most of society and we have been raised in a country with such a puritanical view on sex that we are trained to believe such things are dirty. i knew many girls when i was dancing who thought i was disgusting for wanting to make porn, it boggled my mind...its far less degrading to get paid $800-1000 a day to have sex with a few tested guys on camera, then make 800-1000 (a great central FL dancer week) a week getting pawed by anyone who will pay...but that is just my opinion. i have a former best friend who wont speak to me anymore because my job choice, but that is her insecurity... and her problem, not ine.
i myself dont have that guilt or those views, i am free of them, that is why i do it.... i fuckin' love it. i have an exhibitionist streak a mile wide, and i love the idea of people getting off to me getting off. i get paid to be a slut and do things most girls wouldn't dream of doing, let alone get to expirience the pleasure of...
as for why more guys dont do it... many have tried... few will win. most guys just don't hold up under the pressure, it is a hard job for guys to do, i don't envy the males side of this at all. they work very hard and get paid very low in comparison to us girls.