We should analyse all the different "Avatars" in Smash:
He is the best fit for the moveset Sakurai envisioned for Animal Crossing, since he is the one who uses all those tools. Also his look is very unique and distinct, and he is also quite limited in his customization in comparison to other avatars (for example is face is always a present, you can't change his body structure...).
:ultrobin He is still one of the 3 protagonist of Awekening, and he has his default look alongside an older and younger one of that said default look. But while his look can be customized, his abilities of being a Sword and Magic user can't. That is what drawn Sakurai to him: his unique moveset potential that is common among all player's Robin. Same goes for
.
Their major appeal is in customizing them, which is also why Smash uses them as the "Create a Character" feature. Their customizability has been fully embraced by Smash.
While some consider them an Avatar, they are mostly the same for all player's, the only thing that can be changed is the gear they wear, but their moveset would be the same no matter if this is my Inkling or another person Inklings, since they all use the same weapons (and in Smash they use all of them).
Cross, on the other hand, is completely up to the player. Mine for example was very tall, had a moustache and always wears a smoking, but I'm sure that he looks completely different from anyone else's Cross. And that wouldn't be a problem, but the problem is that he also plays completely different. After a couple hours in the game Cross can select a class from his standard (and extremely boring for Smash) one. That's the biggest problem, every Cross plays differently. Some player's opted to be Attacker, other Healer and other Tanks, and all are extremely different from one another. Creating a moveset using "an attack from each class" just doesn't make sense because of how different they are.
Also it doesn't help that he never had any real impact on the game story, which centered about Elma. He was a "link" (eheh) between the player and the world, but he always felt like he didn't appartein to Mira, and that is because he is meant to represent the player.
He isn't his own character without the player, and that is why Cross is extremely unlikely to be in Smash, especially when we have a real character from that world being Elma.
It would be like adding the Strategist over Lyn from Fire Emblem, just because he is the MU character doesn't mean he is the protagonist, or the most prominent character in the game.