I think, personally, that they are doing their best to keep the Dex from exploding with numbers, but having as many new Pokémon as they can sort of cheat in, if that makes sense. For new players, the "gotta catch em' all" quest gets a little crazy past 700. Alolan forms, Megas, split forms instead of full evos helps give us "new" without overcrowding. I think, if this was several hundred Pokémon ago and a few generations ago, they probably would have opted to make them a split evolution personally instead of a form. These are just my thoughts on the matter, doesn't make it true. I'm honestly not that brothered by it, to me it's such a minor detail that it seems irrelevant if it's a new Pokémon or just a form.
I'm not saying that's the ONLY reason we got things like Megas and alolan forms (I'm sure a lot has to do with marketing and other things. I think several reasons and ideas go into big decisions like this and not just one for sure. They had to do several things as company: make money, appeal to players, they have to have a game model and ideas that are marketable, profitable and sustainable. keep old players and generate interest to new ones). It might not even be a reason at all. But it's just what I think. I certainly understand the frustration of those who think that it needed to be a new Pokémon and not just a form, I'd agree that while forms aren't exactly new, this is a sort of new step for them. Part of it could have been to give Rockruff something to make him more distinguishable. A permanent split form isn't new and neither is a day night evolution, but a split form based on evolving at day or night is. Other split forms are due to gender or something like that.