Tyranitar isn't S-rank because of its slow speed, plethora of weaknesses and bad matchups against all the other weather starters (yes, even Ninetales, as Tyranitar doesn't like a Will-o-Wisp on the switch and Sunny Day+SolarBeam variants will force it out).
Also, unlike Hippowdown, it's Dugtrio bait.
A S-rank pokemon should always be a welcome addition in every team that requires its niche (which is Sand Stream in Tyranitar's case).
However, the very fact Tyranitar's presence isn't desirable in all sand teams (due to the aforementioned flaws) is reason enough not to put it in S-rank.
Notice that the current S-rank pokemon do not have to compete for a teamslot with another pokemon. That's a big part of the reason they're S-rank.
Tyranitar will always have to compete with Hippowdown for a teamslot instead.
Please don't say that Tyranitar is always better on offensive teams and Hippowdown is always better in stall teams as it's a gross overgeneralization.