Well, given that it was a movie that was banking on its fans being familiar with the stars and co-stars, that's the real-world reason we saw Scotty, Sulu, Uhura, and Chekov come back.
As for an in-universe explanation, there might be some prestige in being on the command crew of a refit Constitution-class vessel that was about to embark on yet another long mission.
Scotty may have had something to prove. I would imagine he was integral to the resdesign of the refit Enterprise. And Chekov, as you may recall, did have a different role as security chief aboard Enterprise.
As for Sulu and Uhura, returning in their old roles did seem to be a bit of career stagnation. For Sulu esp., a command-track officer, it would seem that a transfer to another ship as maybe second or first officer would have made more sense for him. As for Uhura, she was probably content to be the chief communications officer of a starship.
Red Ranger