I don't know if Crawford tried to return and wasn't allowed, has that been verified? I figured that since he was from Louisiana, he was tied closely to Mickey. Once Mickey was gone, Crawford didn't have a connection to the team anymore and went back to a school in his home state. With Grimes, I am not sure what happened there, but he has transferred to the FCS level, so maybe he didn't have the desire to play big-time D1 football. I know there are time demands at the FCS level, but maybe he wanted to be at a place he knew he would get a lot of playing time.
With Betts, maybe he reached out to Rhule and the new staff about the possibility of coming back, and Rhule didn't have a negative relationship with him, so Rhule welcomed Betts to come back. I think each situation is different.