I don't get the "X got in so Y can't" argument. It's so black and white. The reality is so, so grey.
1) It doesn't make business sense. If you want to sell as many units as possible, making something the majority of people will want to get is always best because you never know what someone's tipping point is. If the devs can include someone heavily requested into the roster, they will. Ultimate is obviously a game of passion, but, at the end of the day, the dev team and the marketing team have to work together here.
2) It's extremely possible that they couldn't make a character work in a unique, fun way. PP is 100% unique: no one expected it and, from the promotional video, it looks hella silly and fun. While not everyone's cup-o'-tea, look at that dude. Just look at him. 99% of the reaction videos I watched for him were basically, "Oh, PP. Must be an enemy. Wait... WHAT?! There's no way... WHAT!?" And it's not like there was lack of reference material. There are, what? 30 different kinds of Piranha Plant? That is what sets it apart from other generics, like Shy Guys, Slime, Goombas, Koopas, etc. And, unlike all of them, IT'S A POTTED PLANT. It's a **** ton more unique then an anthropomorphic, bipedal character, which is the class that nearly the entire roster fits into.
3) It's not as simple as just developing the character and putting it into the game. Remember when Snake was shown for the first time? Everyone was shocked because, back then, we remembered the ****-ton of red tape that goes into being able to include them. Licenses, rights, demands from the parent company, deal negotiations, potential market analysis. And, even after all that, the character still needs to work in a unique, fun way. It's extremely possible that major requests simply didn't make it through the massive, long process of going from concept to reality. AND EVEN THEN, it still needs to work in the game, else it gets scrapped.
A lot of you mother****ers seem to think there is some sort of alchemy **** going on. No. Just stop. Be salty all you want. Just make sure the salt is in the bounds of reality. Your character didn't potentially lose a spot to another character. The FULL, COMPLETE DEVELOPMENT TIME, INCLUDING THE RED TAPE, didn't go to another character over yours without cause. 100,297.65% sure that characters from the ballot, that did not make it in thus far, likely wouldn't of made it past the parent company negotiations (HELLO SQUARE). That's what DLC development periods are for.
Which brings me to a far simpler, straightforward point: After we had thousands of pages of Grinch Leak discussions (which I was part of team real, mind you), there are people here who are speculating that all DLC was pre-planned already. This is after the devs said it wasn't planned. Like, really? The one thing we should of learned by now is that the devs likely aren't lying to lie. Licensing and **** from #3 takes a lot of time. That's why we have a support date up to 2020.
Finally, I've submitted Sodium Chloride to the FDA for review as a mind-altering substance. Wish me luck.