To be completely fair, that is entirely irrelevant. It has less to do with the individual characters, and more to do with the game they are from. Xenoblade 2 had way more hype and X was largely forgettable. It even comes to the point where I, a Wii U owner that was somewhat interested in X, did not know Elma. Or maybe I did and I just forgot? Who knows, but I was largely not aware of her existence or what she did. I, as someone who bought a Switch solely for Odyssey and future games, such as Smash, and skipped the Xenoblade 2 Direct, knows about Rex.
And not just for speculation. I was aware of him before this game was even announced. I hated him then, for his assless chap, parachute pants and generic anime sensibilities. It is not about Rex's appeal, it is about promoting the Xenoblade franchise in the most competent way, which would not be moving backward toward a largely mediocre game and instead looking forward to a game that had a lot of buzz because it was on the Switch. This is a Smash game made for the SWITCH audience. People keep overlooking this because they are clinging to circumstances surrounding the old game. Those "rules" are just references. Nothing more. This game is completely different and the more apt comparison would be Smash 64 transitioning to Melee. But I was not speculating back then, so I am just relying on common sense here. Rex is a very reasonable choice (even if I hate him). Elma is an outlandish choice (even if I like her).
Beyond that, if they use Pyra, all bets are off. Rex AND Pyra are probably bigger than Shulk, to be honest.