I'll give you a list.
1. Because he's owned by Bungie, a second party company to Microsoft which means he'll ONLY appear on a Microsoft console. Even if Microsoft allowed Nintendo to use him for Brawl (which they won't) Nintendo wouldn't put him in Brawl since it would basically be an advertisement to the XBox, Nintendo's direct competitor.
2. Guns are a big no no in Brawl. Despite Solid Snake barely sneaking past this rule (no pun intended,) the developers discourage the use of realistic guns in the game. Which MC relies on heavily.
3. Samus is already in the game. There's no need for two heavily armored battle tanks in the game.
4. Do we really need Halo Fanboys to get into Brawl?