The democratic leaders knew FDR was at death’s door and they in NO WAY would allow Wallace to become president.
FDR was a worn out old man and had no real fight/say in the process.
What was Wallace’s major problem? It wouldn’t have been being too pro-Soviet; the USSR was our ally in 1944 (that certainly was Wallace’s problem when he ran for President independently in 1948 and only got 2% of the vote)).