I think it's absurd they didn't add more for USUM, even though they added SM incompatible formes (Rockruff, Lycanroc, some totems) and unobtainable Pokemon (the new UBs) anyway, what's a handful of other Pokemon? It's pretty damning for their future.
iirc in some interview Masuda, Ohmori or Iwao (reasonably sure it wass Masuda) said there weren't new Alolan forms because it wouldn't make sense for there to be new alolan forms in Alola (but somehow it makes for there to be 100 more Pokemon in Alola)
