I looked it up and I think it's because Uchi is quite a new personality type and the Welcome Amiibo cards consist of old characters such as Tomato and Gonzo.
I think it is quite annoying because I find the Uchi Villagers to be the most useful, and the Uchi dialog is my favourite out of all the personalities. x
Yeah it's cause they are old villagers. But I do still think they could have made some into uchis... I mean the uchi choices are pretty bad as it is lol.
Pretty much already been answered, uchi is new to new leaf so bringing back villagers would mean you'd have to change their personality to make them uchi. I really wish there were more in general though. Most of them dont appeal to me.
Plucky and Ursala seem to be the only two Uchis to be introduced via the WA update. Kinda disappointing, would've liked to have seen more Uchis as well.