Cuz Brawl needs the hype since it'll get trounced by Halol3.
Nintendo knows MP3 isn't exciting, and Nintendo knows they can't build a good website for it either, so Nintendo isn't bothering to spend the money to make a website and start the hype train.
But BWii will be the real test for online success since there's much more going on in that game in an online session than silly laggy fighting matches and fps-wannabe bounty hunting girls.