Who's to say it's not both? Typically in the same industry, pay is comparable and these 5 all seem to get promotions that likely pay more, too - at least IMO.
If you listen to Toto Wolff's Beyond the Grid or High Performance Podcast interview (can't remember which one), they ask him how they keep their best employees from being poached and having the talent structure fall apart. Essentially, he says that he's constantly trying to give people more responsibility, more input, and more growth. Basically the same thing that RBR has offered them. Perhaps it was a better offer, or perhaps these 5 aren't the best of the best, and thus never got offered these promotions from Merc.