Without having looked at the data I would speculate that competition. When competition comes into play you are usually forced to lower your margins. When the Go PRO first came out, AFAIK, there was no good alternative, and thus they could get away with charging a higher price. Now, that there is more competition they have to either lower their prices or raise the specs of the product they offer.

Then again, this is pure speculation and the reason may have nothing to do with competition.