Tastes/preferences are probably the #1 reason, by a long shot. The dancer you mention is probably barely a 7 my book, if we're talking my personal tastes she's a 6 at best. I wouldn't buy a $5 dance from her. I'm not trying to be judgemental or anything, you probably feel the same about the girls I call a 9. I don't think one of our preferences is better or more correct.
It may be somewhat related, but after that it's probably a combo of desperation and the fact that they can be successful. At some of the lower tier clubs, I see women in their 50's who are way below what I'd consider fucking even with beer goggles on. And I probably couldn't get a shot with them at the club if I wanted to, they've got devoted followers. I'm not sure if they're undercutting market prices or doing things other girls don't, but I can tell ya there's some fat/old/ugly bitches that are pulling in serious cash.
Age is gonna be my number three. I don't see that many truly ugly strippers. I see a lot of fat ones, which I chalk up to taste. And I see a lot of old ones, which I guess is taste too. But during day shift particularly, those older strippers tend to be popular with the older crowd. I can't speak to it and I'm sure there are guys here who can, but I notice a good bit of the 70+ yr old customers spending time with the 40+ yr old dancers. Maybe that comes back to taste again, but I can totally see how a 70 something may prefer a 40 something over a 20 something, even if I find the 40 something one unattractive.
As far as pornstars go, a Racquel Darrian in her prime is more my speed. I'm sure there's a more recent example but I never really remember those porn chicks names these days.