Ophie
Salingerian Phony
It's to make it harder to breed them and get more starters. I think it's stupid, as they'll just find a Ditto anyway. I mean, look how easy breeding a competitive starter is. Just make the starters 50/50 already.
On the other hand, Ditto has (almost) always been a late-game or post-game Pokémon. Hence, I think the idea is to make it less likely for you to be able to breed them while you're still going through the game.
The region where you can find Ditto the earliest, to my knowledge, is Alola, at about two-thirds through the game, which is roughly how far in you'd find it in Kanto. (Looks like Johto had it the earliest by far, at two Gym Badges in, though it has a 5% spawn rate, so you'd have to be actively looking for it to find it. Ultra Sun and Ultra Moon also had the Ditto Five sidequest about halfway through the game though.)