Probably because Underwood knows he needs players, he had a lot of his SFA staff around him, and Gentry was young and didn't have a ton of leverage and would take the job.
Having been the Director of Player Development at a major conference school which was wildly successful turning Travis Ford's talented but garbageball-trained leftovers into the #1 offense in the nation, his leverage is different now.
Why have Joey Biggs and Patrick Schulte been announced and Gentry hasn't? It ain't rocket science, Gentry is looking around for an opportunity for a full assistant gig that would be a better springboard for his future.
It sounds like that search has not borne fruit for him. Though you'll forgive me for not celebrating yet when Evans was also a fait accompli pencil-in for the staff until about 12 hours before he wasn't.