I do believe there isn't much of a emotional reason against them, or a desire to keep Smash about video games (that's still possible with multimedia characters), but it's more of a pragmatic thing. Let's just say getting the rights to them wouldn't be easy, or at least much more difficult for the same payoff compared to most VG characters. I readily admit that when the day comes, SpongeBob will probably replace Banjo or Frisk in my top 5 (Crash, Arle are absolutely not, Paper Mario is maybe). But today is not that day, and I doubt that day is coming anytime soon.
Now back to Plants vs. Zombies, since there's a time or a place for everything but "Anime/Cartoon/Film/Literature characters in Super Smash Bros." would be a very risky topic to talk about: there WILL be arguments if we let it go on for too long.
The reason I suggested a newcomer from it is because I think it's a very popular, well known series that has lots of potential for an interesting moveset, a good stage, a boss, and the music is phenomenal, striking the perfect balance between not distracting and fantastic to listen to. I understand that EA is probably not going to be the guys that open the door to western companies, but I do think that it's at least not on the level of characters born outside of video games. (That isn't a hard bar to pass, though.)