Cuomo's problem is that he is a bully thug even with his own party, Democrats have the long knives out and they are piling on in unimaginable fashion. They know Cuomo is toxic for the Dem ticket in 2022, that's the real reason, self preservation.
Maybe one of the reasons is to position Letitia James, the intrepid Cuomo investigator for 2022 and 2024. Kamala is a dangerous flop--maybe they want to build up a rising star.
One could have argued last spring, when Cuomo was on TV for hours at a time, that he was the great presidential hope for 2024, but that's so... over.