Originally I paired Mudkip and Sobble just because I'd seen people compare the two, and it fit the idea I had for the piece.
I wasn't sure if I was going to make similar pieces for the other two Galar starters. Once I decided I was going to I did give a bit of thought to who they'd be paired with. I considered a few different things:
Chimchar with Grookey, since they're both simians. But I decided against that because I wanted to keep the pairings focused on a single Type.
With Scorbunny I wanted Torchic from the get-go, since to me it was what best fit the race idea I had. I considered Tepig and Litten as well, but I didn't think they would look as good with the color scheme I had in mind.
Once I'd decided on Torchic for Scorbunny I thought it'd be weird not to use Treecko for the final piece, so I went with it. It fit the idea I had anyway, which was a plus.
Sorry for the long-winded reply. In short I guess they were the best fits for the ideas I had.