I think Sam showed up more than once. As for the others deep into the show's run, I think that was the point. Being well-established, they were able to leverage that into making a Cheers reunion episode. It gave them an 'in', making it an easier sell, especially when it was understood that Frasier was a Cheers spinoff. It also meant they didn't have to create special marketing for a standalone Cheers reunion special.
Similarly, with Curb Your Enthusiasm, they were able to get the whole Seinfeld cast for an entire season, where it was more of a show within a show with the actors playing themselves filming a Seinfeld reunion. When a show is already well-established, it makes sense to use what's there as a foundation.