I like the franchise but honestly I think the sales of the original are heavily inflated by Sony bundling the game with a lot of hardware and selling it for dirt cheap very quickly post launch (I got my copy for $24.99).
The fact that LBP2 sold far less than LBP1 says that a lot of people who got LBP1 had their fill of it and didn't buy the sequel.
LBP on the Vita doesn't seem to be selling great either (even relative to the size of the userbase), neither does LBP Karting.
I think this is sort of a case of Sony force feeding the first game to as many folks as possible, but a lot of people bailing out after the first one. To their credit, it is impressive that the people who are fans of LBP are buying a lot of DLC content.