They're on a floodplain.
The floodplain was probably the only place the government would let them build way back when, considering they are black.
Best thing to do now is buy them out.
But market value is low, they're on a floodplain.
If I was a Dallas county taxpayer I would be comfortable paying well above market just to eliminate the blight. And get them into decent homes with running water somewhere else.
This one's actually an easy case, because there are wealthy areas nearby that can fund the effort.
The harder cases are up in the mountains, like that town in west virginia near the old battery recycling plant where nearly everyone gets cancer. There's no one left with any money to buy them out. That's a sadder case, to me.

---