>>22374164A lot of reasons. Greninja was already quite powerful before the release of ORAS. Protean is a rather powerful ability, that essentially makes him a LOT more powerful than his base stats would indicate. Couple that with a ridiculous movepool (Hydro Pump, Ice Beam, Dark Pulse, Extrasensory, Hidden power, Grass Knot, U turn, Shadow Sneak, Water Shuriken, Spikes) and you've got a very versatile pokemon that is capable of outrunning a large portion of the meta game and smashing them with a coverage move of choice. Then ORAS came around and Gren got a few new tools (Low kick and Gunkshot via move tutor) to use that made it go from a top OU threat to a "Gen 4 Garchomp" threat (i.e. overcentralizing the game to the point that every team packs him and whatever few checks/counters for the sets that he runs which are most useful/popular).
The most common set run is Dark Pulse, Ice Beam, Gunk Shot, Hydro pump. Not much can wall this set alone, but on top of that if you want to, you can run Low kick so that even ferro and AV Ttar can't stop him. Basically with the way he's currently being run, there are only two TRUE counters to him are Chansey, and Porygon 2. Of course, you could always opt to run taunt over a coverage move, and then those guys will be completely fucked over.
