Okay, let us clear something up. Adding a character to smash bros is not adding a character, nor a course, to Mario Kart.
Adding a course or a character to Mario Kart affects very little, it is a standalone event, which does not need to be programmed in tandem with anythinges else- meaning, it can be extremely easy or difficult, it does not need to play off of anything else. Balance is no issue as long as it plays well, or controls well, and is fun.
This is not so for Smash bros. Adding a character to Smash is much more complicated- for many reasons. The fist principle is that they actually... do things, unlike in Mario Kart, where they are essentially a face, in smash, you control them, in so many aspects, they have real life to them, not just as an image with weight properties. Yet, this is not the hard part.
Sakurai has stated before that adding characters to Smash is in fact, character multiplication. Not addition. Why is this so? If we have 50 characters, and add one more, is that not addition?
The answer, is no. All characters need to be balanced versus each other, o when you add one more character, you need to play it against each of the other characters. It is not simply adding it into the 50, it is playing it against each of the other 50 characters.
VISUAL:If we just have three characters, Mario, Link, and Pikachu, there are only a few matches of 1v1 to worry about. Mario-Mario, Mario-Link, Mario-Pikachu, Link-Link, Link-Pikachu, and Pikachu-Pikachu. If you add in ONE character, Kirby, you get four more options. Kirby-Kirby, Kirby-Mario, Kirby-Link, Kirby-Pikachu. Then, add in Samus, and you get Samus-Samus, Samus-Mario, Samus-Link, Samus-Pikachu, Samus-Kirby. This is without factoring in 3 or 4 player matches at all! It is actually a very strenuous process, and why a 4-player fighting game of such freedom is so hard to balance.