I'd have to blame marketing for a god bit of it, or rather, their change from disc to digital without changeing their marketing. With a disc, it's not as important because people still see the game when they go to the store. That's what got me into vanilla. So the majority of people who bought the expansions, which were only distributed online, seem like they'd be the people who already bought vanilla in the stores, and thus the following wouldn't have been much bigger.
There's also the fact that, whether there were other attempts made or not, I only ever saw anything about Rebellion here on these forums(Meaning targetting the existing community), Facebook, and a few youtube videos. There may have been a few other things out there, but I never saw them.
Basically, I think going to digital only was the biggest mistake. Think about it: You walk into a store, and see Sins of a Solar Empire, Entrenchment, Diplomacy, and Rebellion on the shelf, and think to yourself "I've never heard of this before...and they made so many expansions, it must be at least moderately good...Oh, look, this Rebellion game here says it includes all the other games features in one...well, since I just have to buy this one game to get everything up to date....." And bam, new potential online fan.
-Twi