A few comments:
After the war Grant said that Sheridan had been his first choice but in fact the man he wanted originally was William Franklin. Little Phil was a favorite - almost a pet - of Henry Halleck and Grant was persuaded to take him instead.
Nobody was lobbying for Buford, for whatever reason, and Buford wasn't Grant's second choice, Sheridan was
In hindsight, to us Buford was the far more obvious and logical choice, like Logan for McPherson Corps in front of Atlanta.
And actually, its not much of a stretch to envision Sheridan, who was an early favorite of Sherman's, getting that corps instead of the unremarkable Howard.
But Sheridan, despite gaining most of his reputation as an infantry commander, had first commanded the 2nd Michigan cavalry and had really kicked butt, which everyone - particularly Grant and Halleck - knew very well, so its not like he was a stranger to mounted service.
I'm a huge Buford fan and I think he would have been superb but Army politics and personal.experience trumped him.