For the same reason some guys like S&M, fetishism, bestiality, bondage, erotic asphyxiation, food play, role play, public sex, urophilia. I think the majority of BMs in this Forum are fairly conservative in their sexual tastes. If you live in a place like Pattaya for long enough, it's easy to get bored with straight sex and easy to experiment.
But of course it's not for everyone.