It was a wartime party unity move by FDR to not make a forced stand of support for Wallace in 1944. Being President, FDR could certainly have shut down the Anti-Wallace movement, and he would have been broadly supported in doing so: 65% of voting Democrats according to the
Gallup poll taken at the time of the convention supported keeping Wallace on the ballot.
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.