It always annoys me when that happensI wonder how much of this is exacerbated by the fact that Ultimate had a shorter dev cycle than previous Smash's. Because only Splatoon had time to prove that it had legs, the rest of the additions were focused on fulfilling long held wants and adding characters from ever green franchises like Fire Emblem, Pokemon and Animal Crossing.
If the game cooked for another year, I'm sure a lot more 1st party requests would have been fulfilled and maybe stuff like Spring Man and Rex.
But I have to imagine Nintendo saw the metrics for the Smash 4 DLC and noted that Cloud, Bayonetta and Ryu sold better than Corrin. Hence the hard pivot into new franchises. If Splatoon hadn't made it into the base game, I'd wager that's the kind of franchise that would make financial sense to add to a pass. As much as I love Dixie and Porky, they're the fourth and third reps for franchises, immediately making them less "exciting"