Dialga G happened....
Machamp happened....
Spiritomb happened....
Rampardos happened....
Gyarados happened....
Simply put it was just overhyped and phails against a lot of decks...
Especially Gyarados which doesn't attach energy to Gyarados anyway so that ruins one of the attacks.
Besides, you are basically putting damage on yourself. Mightyena might be fast and all, but the fact that you are putting damage on Mightyena is a pain, since he doesn't have a lot of HP. Take that, and how Flygon is now in the meta. Then you'll be in some big trouble.