Because we like to enforce stereotypes.

I'm kidding.

I'm gay and I can understand what you mean.
I prefer female singers (though not necessarily in generic pop music).

I like Evanescence, Within Temptation and Nightwish... all female fronted symphonic metal bands.

My other favourite artist is Pink.
So I understand your point.

But I also like Patrick Wolf, Hurts, Maroon 5, Coldplay, Fun.
So I don't really think it's an accurate statement. It's a shame we have to collate a gay person to liking a certain type of music, or a straight person - anyone in fact. I don't think anyone would ever say "why do straight men only like ____" etc. I'm sure there are lots of straight men who like female singers.