Why isnt Mewtwo in ssbb

There's no real definitive answer to this without asking the developers directly, but Mewtwo was likely excluded from Brawl to make room for other characters. Hacks of the Brawl ROM have revealed unfinished Mewtwo data, along with several other unfinished characters who were ultimately not included, such as Roy, also from Melee. In the end, Mewtwo's moveset was likely too similar to Lucario's and the developers wanted to introduce new characters more than they wanted to keep in Mewtwo.