@JGoldman10,
As I said before, you do have a skewed perspective on what is exactly popular.
Waluigi has a dedicated fanbase, but it is relatively small compared to those of many other characters in Smash Bros.
It would be pretty easy to replace Waluigi's assist trophy or even remove it all together if they decided to have him as a playable character. It is not like his assist trophy is essential to the game.
As for Bowsette, it was briefly a popular creation online, but it was by no means a worldwide cultural phenomenon.
Meanwhile, Frozen (two highly successful films and billions made through merchandise) and the Marvel Cinematic Universe films (22+ films in under 12 years, all successful) are actual worldwide phenomenon.
No one doesn't really talk about Bowsette anymore. People, however, will still talk about the Marvel films and Frozen, and will quote favorite lines/songs from the films ("I am Iron Man", "I see this as an absolute win!", "The cold never bothered me anyway", "Do you want to build a snowman?").