Smash Ultimate is a graveyard of abandoned characters.
I did some objective and totally scientific analysis by first thinking about the last time I saw one of these characters in their own series, and then thinking about the last time I saw these characters in a good game, and then looking up when those games came out. Lines are blank where I plain couldn't think of a game that even qualified. (Edit: Some re-releases are included, some excluded. Kinda variable on that one. If I thought of it, I included it. If I forgot about it, I didn't. Like I said, this is all very scientific.)
So excluding jokey-joke characters like ROB and Duck Hunt dog, it's been about 7 years on average since a Smash Ultimate fighter was in a game that was meaningfully about them, and also about 7 years on average since they were in a game that I may have given a **** about (even if I didn't necessarily play it).
That's ****in forever.
Here's some perspective. That means, for all the kids and teens at the widest part of the customer acquisition funnel, i.e. the future gamers, for their entire conscious existence most of the characters in Smash Bros Ultimate have just been Smash Bros characters. They've never known these characters as anything else.
So is this good marketing, or bad marketing? These brands are good enough to make it into Smash Bros, so obviously Nintendo et al see some value to them. But they're also not,... you know, ****in using them? Cross-marketing these characters maybe helps build up some interest (Melee is the only reason Fire Emblem games were ever released in North America), but at the same time there are perfectly serviceable brands that are basically rotting away on the Smash roster.
Nintendo seems committed to pushing their characters into the same niche as Mickey Mouse and Loony Tunes, some-day Nintendo World mascots they can use to sell hats, while their real business evolves into slime guns and phone games.