You asked for vica versa, so I'm going to assume you mean why are bbw/thick white women attracted to black men. If not, please feel free to correct my assumption...
Of course, I cannot speak for all white women, but as a thick white woman I'm speaking on behalf of myself becuase I am powerful attracted to mature, tall, educated, highly intelligent, hopefully hung, black men. The primary attraction for me is simply that there is something unutterably sexy to me about a man - when society in general tries to marginalize and nullify his existence based on nothing more than skin pigmentation - who has the presence of being to live his life in such a manner that his ACTIONS say, "Fuck you, you're not going to define me, nor will I allow you to keep me poor and ignorant, or rob me of my masculinity."
Another thing that turns me on is the contrast between black and white. There is something profoundly erotic about being on my knees taking my lover's cock from behind, and looking back to see his big black hands grip and squeeze my round white ass that makes me absolutely WILD!!! I think it is the play of reciprocal energies that produce that electricity within me. Positive and negative. Black and white. Masculine and feminine. As I've said before, it's the ultimate Yin and Yang!
To me the ultimate act of self-esteem and self-love is to NEVER let a man touch me who does not fully and deeply appreciate every aspect of my being, whether it's my tits, my ass, or my mind. So,another reason is because white man, who typically find me intimidating as hell anyway, will ignore my mind altogether and make sure to let me know okay with them that I have a big ass, as if they're granting me permission to exist, or expecting some kind of humanitarian award for lowering their standards to be with me, whereas the black men who've been attracted to me LOVE and appreciate my body and my mind "AS IS"!
It's also interesting to note that since I started dating black men exclusively, I have been told by countless white men that, "No decent white man will want you if you date black men." To which I inform them, "Well, that's a win/win because any man who is so ignorant and intolerant in his thinking, and insecure in his being, that that actually matters to him I wouldn't want anyway!"