Aside from the physical differences (like tightness), I'm going to guess the reasons are mostly psychological. The level of submission and intimacy is greater than just regular intercourse. The fact that so many white women are reluctant to surrender it to their white husbands or boyfriends probably makes it even more appealing to Black men. There's a connotation of possession or ownership that goes with it, too. If I ever convince my wife to move beyond interracial fantasy into a real life encounter it's one of the things I certainly want for her to experience.