Why?
Answer: Because FDR picked Wallace in 1940 to be his VP, at a time when he was aware of his disease and the prognosis.
If FDR was not aware of how far left Wallace was, then he was not as astute politically as one might think. If he was aware, then the contention that he dumped Wallace in favor of a more conservative Truman is weakened.
On the other hand a lot of things changed in the world from Nov. 1940 to Nov. 1944. Perhaps his dumping of Wallace was due to a more realistic view of the world.
For the 1944 election, with the GOP being so weak, FDR had to have known that whomever was on the Democratic ticket would win the election.
So, the real choices were an ailing FDR or a Socialist Wallace and at least with an ailing FDR, we got Truman.
But of course that does not explain why Truman was not in the inner-circle of the FDR administration, but neither was Wallace.