Because it's humiliating to straight men, and puts women in the "power" role. That's why it's so favored, it gives the middle finger to straight men. And the perfect reply to anyone who doesn't want to do it: "A real man doesn't feel threatened by this and is secure in his masculinity. Now, bend over or you're a pussy." Works every time. It's beautiful.

Just that act. Anal play is a lot better with other things, such as fingers/hands, as well as just simply holding the toy, since your hands are more limited than your hips.

Now, why is it fine with genitals? Mutual pleasure, as well as more awareness of your own body.