I mean they had convergent evolutions for a long time. They just don’t call them convergent evolutions (or community didn’t). In fact you can argue it was there back in Gen 1 with Kabutops and Scyther. 2 different species of Pokemon and things they are based on but are extremely similar and look pretty similar in some regards.It's as odd as the existance of Nidoran m/f, aka remnants of previous gens where a given concept wasn't present.
Possibly when they made regional forms they didn't really think of the concept of convergent evolutions yet. That's really about it.
There are also things like Furret and Linoone, Sentret and Emolga, and a huge chunk of Unova being new Kanto.