Garchomp has some serious advantages over Haxorus. First off, he actually has a really good base speed, whereas Haxorus's base speed means that he'll almost never be able to pull off a sweep with Dragon Dance. Chomp is also resistant to Stealth Rock, has Sand Veil, has more bulk, pulls off Sub+SD sets better, etc.
Also wtf everyone stop saying that you can use your ice types to counter chomp. THEY DON'T. Yes, this is a gen where not everything can have a hard counter, but if you're relying on say, weavile, mamoswine, or frosslass to "counter" chomp then you have an extremely garchomp weak team, since none of them can switch in, and they all outright lose to yachechomp. You're using Lati@s to check it? Fun stuffs, they lose to Haban Chomp or SubSD Chomp if it gets in a sub. Etc, etc.