I feel like it's partly because of all the new Alolan Forms to fully carry out this awkward reboot they are doing. Like when you think about it SM is kind of a unique sequel to XY and follows a similar pattern to GSC (Think of Mr. Pokemon as Hala, Elm as a buffed tropical version of himself, i.e. Hala and Samson Oak as literally the same thing as Samuel Oak was in Johto. Also keep in mind that is a carrying story arc from the previous games, instead of Team Rocket it's Zygarde and perhaps Team Flare/Lysandre/AZ asking it to remove his immortality, so on whatever.) As such, the small number is to promote the Alolan Forms like crazy but also have around 150 Pokemon overall for these generations, and from there go with about 100 new Pokemon each generation, or perhaps for Gen 8 and 9 have about 50 each so that by Gen 10 we start on number 1000/1001 or 1002. Plus of course it would make sense for them at this point to save Johto Starters and their Megas for the Diamond Pearl Remake after all the haters had their little hiatus. I just hope by that point ANYTHING has been introduced to stop Landorus, Thundurus, Heatran, and Cresseila's reign over the Meta. Like some crazy ass Tapu Mon with a pokemon like Alolan Raichu that makes Water types immune to Electric Type Attacks, hella fast, and able to obliterate Cress as well. Kinda like the whole Yveltal -> Rayorge -> Xerndon but not as limited.
