Because back when the XY anime was airing the writers didn't give Ash a Mega for some unfathomable reason, but DID give him Greninja, who was uber-popular. So Gamefreak and OLM created a unique form for his Greninja to face off against other Megas. Gamefreak added said Greninja form to SM later, and it was OU for a long while, capable of hitting against other Megas just as well as it was advertised.
Later they did their best to retcon the form away for some ungodly reason. Just own up to it. In Pokespe Yvonne's Greninja also has the same ability, so while it never transformed, we could've seen a Greninja with Serena's hair.
