While the Western market is obviously a major factor for sales, Smash is a game made by Japanese developers first and foremost for a Japanese audience. There are only 4 characters in Smash created by non-Japanese developers (
), and all 4 were made for a game Nintendo was publishing themselves. That's not to say Western-created third parties are impossible, but I advise folks to keep their expectations in check.
Stuff like Rayman, Assassin's Creed, Fallout, Shovel Knight, Undertale, Shantae, Cuphead, and more are all in the game... but none of them got full playable characters. It's hard not to start seeing that as a pattern - one that is not necessarily unbreakable, but a pattern nonetheless.