It could be because of the PS4 bundle, which was announced before LBP3 was released, but until we see some figures on how that bundle sells over the holidays, there might not be much of a correlation there.
There's also the digital sales issue, although I see no reason why LBP3 would sell any fewer retail copies than any other game.
The VGC numbers could be off, although it's doubtful they're off by a significant amount, same any other title. I'm willing to wait for Sony numbers to confirm how well LBP3 sold.
And of course, this could just signify the inevitable end of the franchise due to a lack of consumer interest, much like Resistance 3 was the end of what started out as one of the bright spots on the initial PS3 line up and eventually faded into mediocrity in terms of market relevance.