Because PS4 is not a beast so it launched at a very nice price range.
Mostly though it nothing to do with power and everything to do with games+other features.
N64- cartridges being too small, arriving late to the party
Xbox- New brand, lack of established games, ugly box= uphill battle
PS3- Price, which is more to do with bluray then anything else.
Graphical prowess is always a competitive edge, but it can't compensate for lack of support and unreasonable pricing.