Just to be clear, Tyranitar is not, currently, hard to counter.
-1 252+ Atk Choice Band Tyranitar Stone Edge vs. 252 HP / 240+ Def Landorus-Therian: 144-169 (37.6 - 44.2%) -- guaranteed 3HKO after Leftovers recovery
0 Atk Landorus-Therian Earthquake vs. 80 HP / 0 Def Tyranitar: 272-324 (75.3 - 89.7%) -- guaranteed 2HKO
I've played about 6 battles of RSE OU, so I can try to explain why Tyranitar used to be better. (it peaked in RSE and DPP and was still pretty good in BW, so this'll be mostly about RSE and DPP)
First, obviously, Landorus didn't exist back then.
Second, sand stream created an indefinitely long-lasting sandstorm. Players didn't have to worry about switching in on the sandstorm's 5th turn and losing it on the 6th. Like the Boxes person said, sandstorm raised Tyranitar's special defense in DPP.
Third, Tyranitar had two good attack stats and a lot of attacks, so it was able to hurt almost anything that tried to switch in. It had earthquake for Magneton, Metagross, Jirachi, and Heatran, fire blast and fire punch for Skarmory, Scizor, and Breloom, crunch and pursuit for Starmie and Gengar, and ice punch for Dragonite, Salamence, and Flygon. Hidden power's max power was 70 back then, so it was even able to nail Suicune and Swampert with grass hidden power. I didn't list rock slide or stone edge. Tyranitar was so good at using those two moves that Pokemon weak to them almost never became common threats.
Fourth, fighting attacks were bad. In RSE, close combat didn't exist, no fighting Pokemon could learn superpower, and high jump kick was almost as rare. Tyranitar was strong enough to survive the occasional brick break. DPP introduced close combat and made superpower more common, but there were still a lot of flying Pokemon and ghost Pokemon that were able to easily switch with Tyranitar when Tyranitar was threatened.