My wife says, for her, there are 4 reasons:
1-yes size is a big part
2-stamina, she was amazed that black men go for hours with total control
3-black men seem to truly care and put her pleasure first
4-skin color contrast; she seems mesmerized when she puts her white hands on a sexy black man and definitely prefers very dark skin like this guy who she loved so much; plus he was almost 20 years younger than she!