Well, I'm going to be mainly talking about reborn. So first you have the lore issue, I mean wee don't know anything about alolan forms. We don't know the conditions for alolan forms to appear. Why does exeggutor has an alolan form and exegcute doesn't but vulpix has one? Also is it regio bound due through external factors, like a stone or legendary? How long does it take to get the form, 10 years, 100 years,... ? Depending on these factors it would make it impossible for reborn to have reborn forms. I mean the cataclystic events of reborn are only around a decade old.
Second reborn is a game that tries to make the player use pkmn they have never tried. Adding reborn forms to that would be a detriment to it since they would draw away the attention to other mons.
Finally think about the effort to create it, reborn already has a slow update schedule. Adding reborn forms will take long time to sprite, implement and balance slowing down devellopment time even further.