Here are the reasons why Haxorus never made it back to OU:
- The introduction of Fairy types. The sole reason due to which many previously lethal Dragons now rot, and Haxorus is no exception.
- Introduction of Mega Pokemon that can do it's job better.
- The increase in the metagame's speed with the fanfare of new Megas, namely Manectric, Charizard and Pinsir. Thus it was always forced to run Scarf sets.
- It's bulk was no longer sufficient to take hits.
- New and improved priority, in Talonflame's Brave Bird, Pinsir's Quick Attack/Feint etc threatened its Scarfed sets and hampered it's revenge killing/sweeping efficiency.
- Being outclassed by Mega Charizard X.
- Old threats like Mamoswine and other Ice type move still haunted it to boot.
Those are some of the most major reasons, I may have missed some though.
Hope I helped!