Gengar was already very powerful, but this new Mega seems absurdly OP. What with that speed, move pool and the ability that made Wobuffet OU to Ubers once upon a time comes back to haunt us. As such, Astro is right, that Smogon has a better say in this, as do GameFreak, but after those guys introduced M-Rayquaza that fashionably broke through the Uber 'ban list' that it had to have its own tier, it is hard to trust the latter.
So, what's with that insane combination? Well, one thing that strikes to me is the introduction of Fairy types . Normal Gengar cannot touch some of the new Ubers, such as dear ol' Xerneas. Even with that cool Poison typing. Yveltal could have ripped through Gengar without a second thought, almost. So some Gengar lovers on the Board probably wanted the 'good old times', back when Gengars where OU, if not Uber
Then again, another reason is because Geomancy Xerneas was OP and they needed a counter for it. A Mega Gengar is one of the few things that can out speed and OHKO Xerneas before Geomancy. Finally, all the others were out there getting Mega formed so the developers didn't want old Gengar to miss out on the fun. Who doesn't like a twist to the meta game anyway (except 90% of the people and the guys at Smogon and tons of others, but who cares, right?)?