This is why I've enjoyed NASB's spectulation more than this game's or Smash Ult's. When anyone can get in, it's hard to even know where to start,
That said, I don't think people are requesting Cartoon Network characters because they think this is supposed to be a PTE sequel, thus they think there's an obligation for there to be CN characters or something.
I think it's because one, since DC, Looney Tunes, Tom and Jerry, Scooby Doo and Iron Giant (and possibly Lego Movie and The Flintstones), the big animated WB properties that didn't originate from CN are all accounted for, more eyes are going towards CN characters for speculation on what animated characters could be next, given Adventure Time and Steven Universe set the precedent that CN characters could happen.
Or two, they, much like me, just really like the characters and want them because of that.