Tenacity isn't a particularly good stat for most tanks.
What tenacity does is allow you to act continuously, for most tanks this isn't a huge issue. If I get stunned as Sejuani I don't care that much, I still have my radiant AOE damage and strong chase abilities when the CC ends. I also don't need a long time to cast my highest impact ability, if I'm not chain CCed I can ult and generally break and turn a CC chain on me, even if I do get CCed again my ult is so high impact it doesn't always matter anyway. Because of these factors breaking CC early as a tank doesn't often matter.
The champs who want Tenacity are mostly fighters who need constant action because they rely far more on active DPS, often auto attacks. If you CC a Jax he loses valuable DPS while if you CC a Naut he doesn't always even lose anything.