It's lizard-brained moral calculus.
Has life gotten better for poor white folks over the last 50+ years? By and large, yes....just like it has for all other poor folks.
BUT....have they seen that, as they advance (which they've done for the better part of a century), minorities ALSO advance? In fact, those minorities, who historically advanced much less (because of Jim and Juan Crow, etc.), have advanced at even a greater rate than maybe poor white people (as they had more ground to make up). So, poor white people had lives that were 50% "better off" than black people. Those poor people now have lives that are a fair bit better than they were 50 years ago...but now they're only 30% "better off" than black people. That makes the lizard brain angry.
If the reward system gave Monkey A 2 grapes when it accomplished a task, but only gave Monkey B 1 grape when it accomplished the same task, but NOW the reward system is to give Monkey A 3 grapes (1 grape better than before), but Monkey B gets 4 grapes (1 better than Monkey A gets), Monkey A gets PISSED. Even though Monkey A is better off, and has more grapes than before...he's pissed, because that OTHER monkey is getting something more.
So, Monkey A votes for a system that fucking burns down all the grape vines. Because if Monkey A can't be the vastly superior grape-getter, then fuck all grapes, NOBODY gets em.