My assumption: Very few people are willing to vote for a party that outright says that they're for racial segregation. So they invent a mostly unrelated issue that makes people ignore everything in favor of their stance on that issue. Then they can freely implement policies that keep the poor poor (e.g. the tax system, the justice system incl. the outrageous US prison system, and the hole "deliberately spread hard drugs in black neighborhoods" thing), which works to segregate because the recently desegregated are obviously overwhelmingly poor and uneducated. And that's despite the fact that poor white people also suffer from these policies, but because of heavy propaganda and single issue voting they managed to have a lot of those poor white people vote for them anyway.

Though I have a hard time believing that segregation was actually the main goal, instead of just making rich people richer.