For two reasons. Faires and Mega Salamence. In Generation 3 - 5, dragons ran through the metagame like they were free in the provinces of Skyrim. However, due to the addition of the fairy-type, most dragons have considerably gone down in usage. For Salamence in particular, it has seen a nerfing due to the monsters that counter it in OU. Azumarill can OHKO it with Play Rough, Weavile can send it to Mordor with it's STAB Icicle Crash (it also outspeeds) and physical walls such as Mega Diance and Heatran outright block it. It also receives heavy competition as a strong dragon type from Pokemon such as Garchomp, Kyurem-Black and Mega Latios. In Ubers using base Salamence would be outright stupid, as Mega Salamence will outspeed and OHKO, and many powerful Pokemon such as Kyurem-White can absolutely smash it.
Edit: Very sorry, I've been quite busy but here's my analysis. New Pokemon such as Hydreigon and oldies like Dragonite counter it well. Dragonite got a new ability in Mulitscale, which allows it to be bulkier than Salamance and perform its role for longer. Hydreigon, is well, Hydreigon. Hydreigon has more Special Attack and has access to Superpower, which aids it. Pokemon such as Choice Scarf Mamoswine and Keldeo can easily revenge kill it. Although it has been dominant in Ubers, Kyurem, Zekrom, Reshiram now counter it as they can hit it with an insane STAB Super Effective hit.
Hope I helped!
