Why's Windfury Shaman not a thing?
Windfury is hard to pull off because most of the good decks these days will never let you keep any threats on the board long enough to pull off the combo, let alone getting lucky enough to have the tools you need in your hand to complete the finishing move.
Shaman has a lot of builds that can pump out insane burst damage, but there are a few issues with Shaman that make it inconsistent and they apply to windfury builds as well as other Shaman builds.
Shaman board clears come with annoying as hell RNG and overload which absolute kills your tempo. If you don't roll a spell totem and don't roll a 3 on your lighting storm it can be GG just based on that.
Meanwhile Shaman just doesn't have any cards or strategies that cheat the system (yet). Early game cards like mad scientist, void callers, flamewakers, mana wyrm, muster for battle, unleash the hounds can provide early game tempo for other classes that is hard to recover from. On top of that lots of good decks have cards that force you into silencing them or getting punished by deathrattles or other effects early game. Then when you make it though the initial onslaught, the real threats come out.
I think Mech Shaman has become popular because it doesn't get affected by some of the drawbacks of Shaman RNG and can "cheat" the system using the mechwarper to get some strong starts.