While all that you said about Hippowdon is true, Tyranitar is a better choice to offensive teams, wich make up most sandstorm teams. Try to use it without having problems related to coverage or not being capable of even 3HKOing the opponent; I've tried to use it and got disappointed with the results. Hippowdon doesn't fit the offensive playstyle: after using Stealth Rock there isn't much that it's going to do for the team unless in situations that you are facing a non-Grass, non-Ice and non-Water physical sweeper. Many times it can't OHKO even things that are weak to Earthquake due to the lack of investiment in attack or Air Balloon, not to mention that there are still too many things immune to Ground. Sure that many times offensive teams can use one or two pokémon that aren't offensive, for support; but there are many better options than Hippowdon to support the team. Skarmory is a better physical wall that also has phazing and Stealth Rock, and even gets Spikes. Forretress can Rapid Spin and also has all three hazards. Not to mention that Hippowdon gets destroyed by a lot of special attacks without investiment in SpDef; Tyranitar can even sponge some super-effective special attacks. Hippowdon is definitively not recommended for most offensive teams unless you need something that doesn't get wrecked by Dugtrio.
As for Genesect, I don't doubt that it could become number 1 in usage this mont, but that's not what happened, and Genesect already got a lot of usage last month.
I am a 18 year old guy that lives on Brazil. I have my own desires and goals, for example, I plan to become a voice actor. I also plan to become a mangaka.