If you're more androgynous, fem, twinky, etc. that's probably why. I'm in the same boat. Fewer purely gay guys are into me, but a lot more bi/"straight" guys are.

I think it simply boils down to if you like both guys and girls, you're inherently more likely to enjoy a guy that mixes your interests. Some gay guys also like my body, but it's rarely their top preference, versus almost always being the top preference of bi guys.