It was called the "solid south" for a reason.

A lot of Republicans... were dead.

The Republican party had been purged from the South. In the North, the progressives, socialists, and communists made hard appeals to black populations in the early 20th century. This is one of the reasons why WEB DuBois was a rabid communist and civil rights advocate (and note that Civil Rights as a concept is a kind of left-wing idea)

Republicans of the same era (see Teddy Roosevelt) made no explicit appeal to blacks. They simply ran a kind of default American approach. They asserted that blacks would be successful as they developed economic infrastructure, became more educated, became more literate, and embraced Anglo values and behaviors. This is true (see Booker T Washington).

Between these two figures, Washington v. DuBois, DuBois had the full throated support of the Communists and non-White-Supremacist Socialists; whereas Washington only had grassroots support, and had no significant Republican party support. His movement was a major cultural force, but was not antagonistic to Communists. As a result, institutional capture occurred as always.

Since most black Americans lived in the South, and since most black supported Republicans were purged or dead, the only people who could appeal to them were Communists and Socialists who explicitly promised to support them specifically, as blacks. Most of the Dixiecrats were still White Supremacists, so this is where there was serious tension between Racialist Progressives & Black Supporting Communists during the 30's.

White Supremacist racialism really started to become unpopular with the intellectual elites of the political Left by the 40's. This is actually when you see some of the progressive racialists who invented the IQ measurement, reverse course on IQ entirely. The Roosevelt administration is where things really start to tip because while FDR is trying to appeal to Southern-Whites, Elenor is appealing to Southern-Blacks; and both poor blacks and poor whites would benefit from being bribed by welfare measures. (For example, the $25/hr minimum wage for cotton pickers; yes, that's 1933 USD btw. Gold was $35 /oz).

So, if American blacks can't vote Republican without being physically attacked (and because there aren't any), and the Left is beginning to move away from White Racialism, and Communists explicitly seek to support them, and HSBC's are comfortable with Leftwing Intellectualism... well, there's only one direction they can go.