Tenacity is the reason why League has experienced knockup creep in the first place. Tenacity, especially widely-available and relatively cheap Tenacity, effectively means every champion with hard CC needs to be balanced around two time frames, one where their hard CC applies for the full duration, and one where it applies for only about two thirds of the duration. This is obviously impossible, and so many champions who especially depend on their crowd control end up with overly long durations, e.g. Morgana's root, or Fiddlesticks/Rammus's point-and-click fear/taunt. Knockups, on the other hand, are impossible to reduce, and extremely difficult to cancel, so you can basically count on a champion having about the same window of opportunity against any target, provided they land their ability. It's clearer for everyone, easier to balance, and generally allows for shorter-duration CC, so even if tanks can't reduce the time they spend getting disabled, they at least don't have to sit through 3 or 2+ seconds of hard CC each time they get hit (and squishies don't have to watch their champion get killed during that same window just because they didn't buy Mercury Treads or QSS).
Another problem with Tenacity is that it doesn't really work as a typical "stat", because stacking it is horrible for gameplay health: Irelia is the prime example of why Tenacity shouldn't stack, and if she gets enough of a lead to go for the full Treads + Elixir of Iron + DMP + Trinity Force combo, there really isn't much you can do against her. It's also one of the many reasons why Mundo can't really be as strong as he deserves to be, because the combination of his near-free sustain and CC reduction means he becomes almost impossible to truly focus if he gets ahead. Sure, Mundo goes where he pleases, so it's at least thematically appropriate, but it's not really the most interesting implementation of that for anyone (Mundo's strong because he makes moves against him less effective, instead of exerting that power more interactively).
While practically everyone would want the option of reducing incoming crowd control if they were given the option, the problem with Tenacity is that, while it's desirable as a stat, it's not actually fun. It's not fun for CC-heavy champions, who have to deal with reduced clarity on their effects, it's not fun for players who don't have Tenacity, because they have to deal with inflated crowd control durations, which leads to squishies dying arbitrarily and tanky champions getting kited too heavily, and it's not even fun for players who do get Tenacity, because the prominence of Tenacity-ignoring crowd control means the stat isn't even all that good at doing its job. I really think the best solution here would be to cut out Tenacity completely, and instead tone down the duration of more egregious CC effects as a result, with compensatory buffs given to affected champions (they could afford to be more powerful because they'd be defined around a shorter, much more precise window where their target's disabled).