Sakurai:

The most important thing was simply whether or not it would make players happy. I mean, there are lots of popular games with characters that just aren’t meant for fighting and were too hard to incorporate into the Smash Bros. title. Heading the list of these characters are the ones from the games we just mentioned, Nintendogs and Animal Crossing. Because these are games played by lots of people, I knew players would be happy to see them make an appearance. This is why I wanted to find a way to put them in the game and why they became Assist Trophies.