Historically(and somewhat to an extent, presently,) most African Americans live(d) in the South because they are(were) unable to leave because of a system of oppression that prevents(ed) them from getting even a half-decent education, or a half-decent wage to get the fuck out of their prison. Most proponents of racism aren't even going to give the group of people they dislike a chance to leave the area if it meant having to associate with them on a day to day basis. That, and well, psychology. If the only white people you had experience with were hate-mongering sons of bitches, you're going to assume every other white person is(which yes, is racist,) and won't even try to change your situation because of a perceived uselessness in doing so.
It takes money to uproot and transplant your family elsewhere, and if you're not getting a job or lack an education well enough to be considered for one, guess what happens? You're not going anywhere. The Black Civil Rights movement didn't really hit full swing until relatively recently in the last 240 years this nation has been around(or 520 if you really want to get technical about it.) And it's still being fought for in some areas.
But really, just go on thinking "Heh, we have black people everywhere. They must've really liked it here if their families never left," and ignore that racism, historically and currently, has cemented most of them there in one way or another. Yes, things got significantly better for them, but that doesn't mean it's a utopia for Blacks now.
Also, Wikipedia is far from a reputable source.