The reason we won't see a demo of SSBB is because of money.
Nintendo knows they'll make a crap load off of Brawl whether or not new players decide to try it. Word of mouth will spread the game. Simply put, a free, public promotional demo isn't needed.
Nintendo would have to pay everyone involved in the development additional money to use this "new" product available to all the public, because in the game business world, a public demo isn't seen as a trial or an "in progress piece of work"; it's seen as a completely different product for the market, thus Nintendo would have to shell out additional money to everyone involved, on top of the original development costs. Demos have always been done this way, except it is usually planned from the start, so the "additional payment" is already factored into the budget.