Why is hiv more common in homosexuals?

Several reasons: It's more common in gay/bisexual men in much of the developed world, but globally it's more of a heterosexual epidemic. Reasons for high rates in gay men include (1) history: it established it first foothold in that community (2) epidemiology: rapid spread due to multiple sex partners, especially in late 70's and early 80's, (3) biology: anal sex is an easy way to transmit for hiv.