I mean, it makes perfect sense to me- they're supposed to be weekly vendors. The benefit of having them on Harv's island is having access to them 7 days a week (say you need more plant starts from Leif, or decide you actually DO want that rug, or don't quite have the bells to buy something, etc.) and not having to track them down. But they're still weekly vendors- Nintendo didn't want to devalue them TOO much.
Sure, it'd be easier if they restocked daily. But the game already has given us a lot more convenience than previous games, so it seems fair to limit us in some ways.