Keep in mind, Sakurai's not picking DLC as an advertising board for the Switch. We've never seen any statements or content that would suggest that. Plus, the idea of someone purchasing an entire console solely to play as one DLC character in one fighting game seems farfetched. The combined price of the Switch, Smash Ultimate, and a DLC pack in the US is $386, you'd have to be a super-fan of the franchise or character.
Ultimately, Sakurai's gonna pick characters he thinks are cool and have good moveset potential. While this pack has been focused on 3rd parties, I highly doubt he'll deliberately ignore characters who would be fun solely because they aren't hype enough.
Oh, what a coincidence then that the fanbases of Persona, Dragon Quest, Banjo-Kazooie, and SNK are full of super-fans who'd buy consoles just for a single entry.
The entire Fighters Pass so far has been a two-way street of endorsement. Sakurai gets to highlight games that, from a Japanese perspective, don't have as much international exposure as they deserve (such as Fatal Fury or Dragon Quest). Nintendo and Sakurai know they have the power to boost any franchise they want, and Sakurai has been honest in the fact that he delights in that opportunity.
In return, they get the attention of those franchises who were never particularly Nintendo-loyal before (or in Rare's case, forced to migrate away). Nintendo can't
force third-party companies to make games for their platforms, but Smash gives them the opportunity to draw the attention of those companies' fans towards their systems, even if the latest entries are exclusive on competing systems. A person who could only get into Smash because of Pikachu ends up learning about Mario, Zelda, Metroid, Fire Emblem, and all the other great Nintendo-exclusive games; imagine having the power to repeat that scenario with a whole new crowd of players for every new franchise you add to Smash.
Spending
any of Ultimate's DLC slots on a first-party is a waste of that power. And since good movesets can be made out of pretty much anyone these days, then there is no reason for them to squander their power like that.