I think two things:
1. It would be nice to have a VP who can help with either a state or a voting group. Many of those on the list just don't fit that category. Trump is already going to win North Dakota, Florida, Ohio, Arkansas, That leaves just Tim Scott, Elise Stefanik, and Ben Carson. A black can helps with the black vote and a woman can help with women's votes.
2. Trump needs someone who can shove it right back in the face of so called journalists and out of the last three I mentioned, Elise Stefanik is really the only one of the three who can do that.