I mean, it's doesn't just come down to raw sales numbers of the games the characters game from. There's (probably) all manner of ways and reasons Nintendo and Sakurai choose new fighters, from stuff like historical relevance (whether with Nintendo, or to gaming as a whole for more notable characters like Snake and Cloud) to stuff like how willing the other party is for it to happen (for example, if Cloud wasn't, ya know, Cloud, I don't think Nintendo would've fought all that hard to get him in if they were only working with a couple of songs when others -- including Konami no less -- have allowed Nintendo to play around with plenty more).
For those two in the title in particular, I can see 2B being ruled out because she was already a guest character in SCVI (as DLC too, similar to how she would be here) and they want to avoid being seen as just "copying" for want of a better word, alongside some other reasons. For Tracer, while she is the mascot of Overwatch, it's a game with no prior historical relevance (obviously) from a Western company (while there are Western-developed characters in Smash, they are all ones with very strong ties to Nintendo) whose last game on a Nintendo console before Diablo 3 was at the turn of the millennium.
Now all that being said, I do still suspect Persona 5 will make its way to Switch in one form or another, so this is all just academical to me.
