That I do not have the answer to.
Again, by no means do I think it's impossible. My point is though that negotiation and collaboration for these characters, the accuracy of their movesets, their expressions, negotiations over their amiibo and pricing, it gets complicated. Sakurai has noted in the past how difficult it can be to consistently make these characters to the demand of the creator, not just with 3rd party companies but closer companies like Game Freak or the Pokemon company.
Now with all of those things, it's far easier to plan, budget, and implement things if you work with companies that you've not only worked with before, but still have quality characters that have demand, and are easy to travel to.
Although, I will say that the presence of Shovel Knight as an Assist Trophy seems to contradict that, but then again he isn't a full fledged playable character.
As I've said to others, it's a wait and see scenario. The biggest thing for me is convenience.