Where's Jonny?!
It's not Aluminum, but It'll do. >
Nintendo is trying to diversify their franchises (with other games like metroid, zelda, Starfox) and not dilute their fanchise's uniqueness by releasing a sequel (encompassing all the franchises). It's one of those "1 a console" games. If a game is as good as SSBM, it will stand on its own for years and years.
Although a counter point to this is the Mario Advance series (and mario games in general). Nintendo releases these because they know people buy up these games despite the constant mario name attached and there is such a large GBA fanbase (and a fanbase that will buy any game with the mario name; Mario=quality to them), and kids always want something new (which in the majority of the GBA demographic) that a new portable mario is. Now, these games do not have the depth like ssbm and the infinite replayability. SSBM is still one of the top played games on the GC I believe and this is why I think Nintendo hasnt opted for another installment.
SSBM is GC's stand alone multiplayer. It is a game that helped found the Gamecube. You all know it, Nintendo knows it, it's been stated here as well. SSBM was really a showcase of franchises to get all those that might be interested in one or another franchise would get the game and a gamecube. A giant "fan net" if you will. Since this game already is out, why would they release another with the same initial idea? It would be a redundant propaganda tool, with the technology the GC is set up for, they would want to make the next different enough, just not a character and mode expansion which many people want. Why use the same (propaganda) tool again when the system doesnt need it because those franchises now have their own games.
I would be all for a new SSB, but looking at the initial purpose of the game, and what options Nintendo would use to expand the game, it would seem pretty redundant and just a expansion of what already is, nothing totally innovative but new gameplay that would be different, but not with what Nintendo is going for these days. They could invent new modes and other ways to fight, but they want that muli-franchise media tool again, and not over present the game idea of SSB to the public. That is my guess on this.