The reason they didn't advertise is because
a) The game wasn't technically "ready" for any form of sales
b) Only a niche audience of Spore players would bother buying (I for one bought the Special Edition, and I found it to be a waste of money
c) What could they advertise? It doesn't have next gen graphics, cut-scenes of commercial value, or even voice acting within gameplay. It doesn't have the massive array of possibilities Spore had, and even Spore wasn't advertised heavily.
Frankly even now in its death-throws (unless of course they're going to release more...doubtful imho) the game is extremely rough, had EA not ruined Maxis, I believe this game would've stayed in production for a solid year, and after extensive testing and polishing be released as a kicka** game.
Don't get me wrong I still play spore to this day, but Darkspore just doesn't pull me in as much, and the "hero has too many detail parts" bug (Which has existed since the beginning...) keeps me from devoting time to editing Heroes, let alone playing the game.