I'm going to use an example of what you're talking about: Zac right after his rework.
Zac pre-rework was a bit of an ap bruiser. He had decent damage, could spam abilities in a side lane, and generally had a bit more to his kit that was damage leaning vs being cc. Riot decided that they needed 3 tanks to rework when they were doing a "Tank update", and alongside Maokai and Sejuani, Zac was chosen.
Now, Zac's rework was exactly what you described. He lost a ton of reliable damage in his q, w, and even ult in order to get more cc, bringing him down from being roughly top 30 jungle champs in terms of damage to the worst damage dealer in the jungle, below Ivern and Nunu in terms of dps per game. In exchange, Zac's cc was boosted to great levels, making his e + ult a full team root and displacement, and his q made his ganks even better.
Do you know what happened to Zac's playerbase right after this rework? It dropped, heavily, despite him already not being that popular of a champion. It turns out that despite getting more cc in exchange for less damage, Zac just didn't appeal to as many people after the rework. Without damage, he lost clear speed, he lost the ability to win duel well, and he became reliant on his team being able to follow up on combos rather than having any self-sufficiency. To the general playerbase, Zac just wasn't as fun after the rework, despite being tanky and cc heavy.
But the worst of it wasn't that Zac was just less good solo and relied on his team more. Its that this sort of champion could never see good buffs. Right after Zac's rework, he saw a surge in pro play because of his cc and tankiness. It didn't matter in pro play that he lost damage, because pros are reliable teams who can make better use of their champions strengths when it comes to needing follow up. Because of this, an already struggling Zac saw nerfs that made him even worse than before, just so that pro play didn't have him dominate their meta.
This isn't exclusive to Zac; pro play abuses tanks that have decent cc or abilities to the point that Riot makes the champs functionally unviable outside of pro play. We saw this with the 40% winrate Sejuani who saw pro play after her rework. We saw this with Tahm Kench, who is still picked in pro play despite being hot garbage in solo queue. And we saw this with Zac after his rework, when despite having a lower winrate and playerbase, Riot couldn't give him effective buffs without fear of pro play dominance.
In the end, Riot reverted Zac's ult in order to give him damage back into his kit, and that's kinda the nature of these "CC heavy, damage light" champions. They get changed so that damage can be added without fear of ruining pro play, and because cc heavy, damage light isn't appealing to the majority of the playerbase. Its why Sejuani lost cc to gain damage in order to get her out of pro play. Its why Tahm Kench saw a rework to put him top lane, and now that that rework showed he was still just as toxic, Tahm Kench is getting another rework to be less problematic. Its harder for pure tanks to carry games. Its harder to have an impact on games, and to farm, and to not be at the mercy of the enemy team, and that doesn't appeal to as many people as impactful champions.
TL;DR: High CC, Low Damage Tanks don't appeal to the playerbase as well, and they are problematic to balance due to the power they have with coordinated teams like in pro play. Tanks with less cc but more damage are easier to balance and have more people who play them.
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