Couple things are to blame here.
1- LBP 3 launched on the same time as GTA V and the other big titles in the US. Maybe this dosen't seem like a big deal but it seems likely buy one game now and hold off for another later.
2- Digital purchases might be a trend for this game. It's a kid friendly game but we all know some kids are crap at handling game discs. Gets annoying at times switching discs sometimes to put in what they wanna play.