It made sense for Alola to have regional forms since it's an archipelago isolated by thousands of miles of water from every other region, but what about Galar?

Actually, it'd make sense for Galar to have regional forms if Kalos also had some. Kanto/Johto/Hoenn/Sinnoh are all close to each other, in the same "country", so it's understandable that they wouldn't have regional forms. Unova is on the other side of the planet, so instead of regional forms it makes more sense for it to have completely different species occupying the same niche as the Kanto-Sinnoh pokémon do in their home regions.

But why does Kalos not have regional forms? Especially when Galar, which should be right next to it, does?

The concept of regional forms is neat and I love that they brought it back, but the fact the phenomenon seems to have so little consistency bothers me.