And Chrom was going to be. But he had no way to make him stand out at all. Robin got chosen because he/she had a better moveset. The 3 characters in Awakening are not all vastly unique as they could be. Which is why one stood out more than the other.
Besides that, Chrom had the bigger story focus. The Champions have equal story focus. That means Sakurai has literally no reason to look at one at any particular one as "more important". They solely have fan polls going for them at this point, really. (I realized how I didn't specify the story focus of why Chrom isn't in a similar situation the Champions, so... yeah).
This cannot be applied to the Champions who are all unique, but equal. Nobody stands out or is the "main character" in any way over the other. Besides that, the fact none of them are playable nor was there ever any reason to believe one would be playable over another puts them in a situation that makes it highly difficult to choose one over the other. None stands out in some significant way at all. All you can find are polls at best, and that isn't going to be enough of a reason on its own to get any in. I'm just sad they didn't get Assist Trophy roles, really.
The problem is you made a bad comparison to begin with. No Champions is the "main one". Fox is the main one. There is no equal importance. The only groups in a similar position to the Champions are 100% equal characters. A better example to look at is the 6 possible options in Final Fantasy 1. These are what the Champions are like(just playable). That's what they're meant to be. A group of 4. Not 1. Not 2. Not 3. 4.
And that has nothing to do with why Falco was chosen. He had the perfect body type, was not as important as Fox, and also had popularity. The Star Fox crew has always been a group that has changed cast members, with only Fox(or his similar counterparts) being the main character. The only one who almost would've been as close to Fox is Krystal, oddly enough, if she was playable in Adventures like she was meant to be. But she also has more focus in Command, so there's implications she was at some point meant to be the closest to his level of Main Character. That obviously changed.
In this case, Sakurai has never done this type of thing before in any way. When it came to a group of equal importance(which has yet to have any member playable in Smash, with technically the Awakening trio kind of counting, but that was a more unique situation too), the thing that's important is that one has to outright stand out from another. This does not apply to the Champions. They are all highly unique. We don't have a Chrom situation where he's too similar to Marth and Ike to be worth adding, which is why Robin got in instead. The only reason all 3 are in there is because Echoes was possible. That's literally it. If Echoes weren't a thing, it would only be Robin.
That means we'd need to have the Champions all playable at once for them to really stand a chance. Because none of them truly stick out compared to the other. They all have different archtypes, weapons, and abilities. None are promoted over the other. None are practically the same as a playable character in Smash either. They just don't have a remotely similar situation to any currently playable character. The're highly unlikely until they start treating the Champions as more than a group on 100% equal terms with no stand-out factors.