I suppose the difference is Bonnie being generally well liked and her and Clemont being an inseparable duo is why they get to return. Like I could never imagine only Clemont appearing without Bonnie, they're basically a set.While I'm not a fan of Max at all, it is unfortunate that it appears we won't have him, nor May (who may or not may not influence the return of her brother), back in any voiced capacity. If Bonnie can reappear, and more than once at that, then Max deserves to come back too.
I think a bigger question is if Misty or Serena will appear despite not being in the opening. Misty hasn't appeared in a saga situated in Kanto most of the time which is strange given Cerulean City is right next door to Vermilion. And of course Serena is the big question.
Maybe Dawn, Iris and the SM cast really were the only planned female companion returns and people jumped the gun assuming all of them would come back. If we really don't see even Misty by the time this saga ends then I assume they only wanted to bring back a select few characters, Dawn for DP games, Iris for the PWC, and the SM cast because it's difficult to visit Alola without showing them and they were the most recent series