Here are some reasons:
1) Although it's changing, most lesbian porn kinda sucks. It's made for men, with plastic Barbie doll pornstars who usually aren't even bisexual, let alone lesbian. That porn lacks the passion and chemistry of straight and gay porn.
2) For lesbian women who are part of the butch/femme dynamic, straight or gay porn most often resembles that much better. If you're a butch woman who likes getting her strap-on sucked, for example, it's very easy to mentally place yourself as the guy in some gay or straight POV porn. It's easier to relate to than femme-on-femme porn.
3) Gay porn is more visual. When you're with a woman in real life, you can feel and smell and touch her arousal. In porn, it's a lot more difficult to tell if the woman is faking it, as is so often in porn. You can't do that in gay porn- the guys have visible erections that prove their arousal, and the visual pop shots of ejaculation. It can make the porn feel a lot more authentic than lesbian or straight porn.
4) In straight porn, there's often a lot of misogyny and anger, which can be a turn-off for more feminist-minded lesbian women. Gay porn, on the other hand, is supposed to be more equal and egalitarian- two guys doing what they want.
5) Queer is queer, baby. Lesbian porn is made with a straight aesthetic- despite being two women, it fits into ideas of heterosexuality. Obviously straight porn does this as well. Gay porn tends to appeal to the queer aesthetic of many lesbian women.
6) It's kinky and taboo and different. People like watching porn that doesn't reflect their real lives. Ergo, gay porn!
As for why gay men don't watch lesbian porn, well, it's not entirely true. I posted some lesbian videos on Xtube and had a surprising response from one gay guy who was shocked at his own aroused reaction! I'd also add in the aforementioned #1- lesbian porn is made to appeal to straight men and their aesthetic. When you have lesbian porn that's actually made by lesbians, for lesbians (like Crashpadseries.com, for example), gay men tend to be a little more responsive. And you can't forget the rampant vagina-phobia that runs in the gay community; there is no similar penis-phobia in the lesbian community. I'd call it more disinterest than actual fear or disgust of the penis.
Hope that helps.