But Goku is not principally a video game character. He's a character that's been in video games, but he was not conceived as a video game character.
I think of it this way: Smash is a fighting game for Nintendo characters. The third party additions can be considered "Nintendo-adjacent" because they're all original, primary video game characters and aren't much of a stretch. Someone like Goku is a massive stretch because he's neither Nintendo nor adjacent, and so shouldn't be considered for Smash.
Goku might be popular, yes, but would putting him in over someone a lot more recognizable, especially a video game character, really sell more games? I don't think so. They wouldn't lose much, yes, but it would dilute what Smash is as a brand.
If they put Goku in, what's to stop them from putting everything popular under the sun in? It wouldn't really be Smash then.