Why boys?

Why are the vast majority of the victims boys? You would think that young girls would be equally or even more attractive to pedophiles, but that doesn't seem to be the case.
Could there be some biological factor involved? Perhaps a hormone? Man/boy sex goes back at least to Sparta. Is there more to it than abuse?
Could there be some biological factor involved? Perhaps a hormone? Man/boy sex goes back at least to Sparta. Is there more to it than abuse?