Because it's rarer and in turn purer.
It gives a nice and complete outline of the body without showing loads of skin, so it's more of a subtle sexy than a look at me bikini. It also makes for a more distinctive presentation of the legs, since it switches between covering the hips and body but showing the legs. Makes it good for legfags like me.
I like how "glossy" they look, and inevitably in every doujin when the swimsuit gets grabbed and pulled and goes between their pussy.
I can agree to that. But I don't know why it's usually the other way around.
No way, I prefer a flat chest/small breasts in a one piece.