I know what you mean by saying why they're chosen in Melee/Brawl (By they I mean Young/Toon), but to be fair in Melee they(Clones) were added to bump up the roster and Brawl replaced Young with Toon. At the state Smash is in now, it has already included the key characters from their main series (Excluding DK & Metroid), if they were to add anymore characters they have to go for lesser known characters. Take Rosalina for example, she's gaining a lot of popularity now, but she isn't a key character to the franchise at the level of Peach, Luigi, or Bowser. From Sm4sh forward they need these characters if they ever think of adding more reps for these franchises. They could keep adding variants of the same characters, but that wouldn't represent variety for Smash, which by recent interviews we know Sakurai is going for.