Some guys aren't interested. I think they're a minority, but my guess is, anyone dating interracially probably has a slightly lower success rate, just don't take that to heart.
More significantly though, I'll say two things (which have the same result): - just like you're not sure if white guys like you, white guys aren't sure if you like them. In fact some stuff I've read suggests a larger percentage of black women aren't interested in white men than vice versa. But I know plenty are. Anyway, the white guys you're eyeing? Many of them assume they have no chance. - there are different cultural expectations about approaching and indications of interest etc. These aren't by race, but lets be real, there's some overlap. In north america, the average white guy is a little more subtle than the average black guy (and less persistent) while being MORE obvious than the average Asian guy. All these 'approaches' are fine within their own group, but can be mistaken outside.
Basically white guys are gonna be a little less confident and a little more subtle even if they are confident.
I'm not sure this is a race issue, you're a very pretty woman and guys do get intimidated. Most of my white friends aren't the type to walk striaght up to a girl (white or black) and ask them out, they prefer to make conversation somehow or be introduced by a friend. I'm sure if you spoke to the guys who are checking you out they'd be very pleased ;)
I'd think you'd have no trouble finding a white guy. Seriously... you are very pretty.
A lot of white guys believe black girls dislike them. If you make eye contact with a guy you'd like to chat with smile at him. Let him know you won't bite his head off if he tries to interact with you. Girls on average may be smaller than guys but that doesn't mean y'all don't scare the hell out of us...