Oh, it's not just that they have to sell less than any random franchise in FP2, it's that they have to be less popular than all of them. Just like how FF is less popular than Minecraft, Tekken is less popular than FF, XC is less popular than Tekken.
I think the divide is that Smash Brothers includes fans of multiple genres and eras. For example, I would prefer characters that have been in gaming a while and originated in Arcade, NES, SNES, and N64 eras because that was my childhood. By the time Gamecube came out, I was in Highschool.
You also have multiple nations and regions of the globe that play Smash.
For USA region, characters like Doomguy, Crash, and Masterchief make ALOT of sense because they are iconic figures to us for gaming but maybe this isn't the case for Latin America or Japan. Meanwhile a character like Joker is not going to be iconic to me because I really don't play Playstation JRPG games. He doesn't fit my region and the time that I prefer in gaming. So of course, I would naturally want a Crash or Doomguy over him. However, I must respect that perhaps he is iconic to other people.
I never owned an SNK title either and played Terry. So I have no relation or affinity towards him.
Minecraft came off as childish to me (heck so did Pokemon) so I had no interest in it but need to respect it is one of the highest grossing franchises of all time.
I saw very different list of characters that should have been in the game based on my narrow view points and we are all guilty of this. The key is to just not expect anything and don't let all of these leakers hype you about a Crash being in the game when it won't never happen.
Heck, when I was a kid, I would have been more hyped for characters from Diablo, Starcraft, Ghostbusters, and TMNT being in the game
. I have moved on. I got Sonic in the game, I got Megaman in the game, I got Pacman in the game, I got Simon Belmont in the game, and I got Banjo in the game. I am happy. With the exception of Megaman and Sonic, most of the most iconic franchises that I played as a kid have been represented all of the way back as Smash 64. Mario is probably my favorite all-time franchise and LOZ and Kirby are up there as well. Sonic is probably my favorite third-party franchise followed by Megaman.