I have seen Ben Carson speak at a physician convention. He spoke well, saw no problems with that and he had some funny jokes.
Neither is he charismatic, to win them over with aplomb.
When he ran in the presidential primaries in 2016, he not only had almost NO support, but everyone else, EXCEPT TRUMP, snubbed/ignored/was rude to him.
This time around, Trump needs not only someone loyal to him and MAGA, but someone who can get elected in 2028.