At first hardly anyone played Paladins. Then they were given melee crits, bonus melee damage to more or less everything from an AA, and self-heals.
Then people realized that Paladins had been primo AE tanks the whole time--it just hadn't been a thing before people started playing pallies and getting good enough gear to tank multiple things outside of Eldenals. Eventually it was discovered that group heal aggro was more reliable than the resistable AE blind, so group heal aggro was reduced a lot.
Then aux tanking was introduced and made to apply to players, so tank melee damage was always reduced, but especially when AE tanking.
Then aggro from weapons was improved to make Warrior aggro more reliable in general and to make one of their AAs better.
Then knight 1h weapon ratios were nerfed (hi) to be in line with regular 1h as a general itemization improvement and people stopped playing knights forever. And they were also given a damage boost when having a shield equipped and an aggro boost on bash soon after.
Then Hexdoll and other ammo slot pulling items were nerfed and everyone stopped playing Warriors forever.
Then Warriors were given foelock to make their snap aggro more reliable.
Then knights were given a damage bonus to 2h weapons from their 2h AA to make melee dpsing slightly closer to a real option.
Then Shadowknights got a buggy restrictive AE aggro thing from Cyzaine cuz why not I guess.
Then Shadowknights got a new tap and new line of mostly crappy reverse DS things from me cuz why not I guess.
Then Shadowknight aggro spells were changed to save their fingers and spell slots somewhat, and because Cyzaine isn't very thorough I think the stacking/overwriting can be abused to get more aggro early on than intended at the cost of not really saving spell slots but who cares cuz SKs were always about not having to worry about aggro.
Then maybe some other stuff I can't remember.
Then Warriors got a new system cuz they were boring and I'd been wanting to change all of the tanks anyway and the warrior idea was the least radical. Warriors got some AE aggro and DPS power out of it that they hadn't really had before, slighting Paladins.
Then Paladins got a slight boost to a couple spells for aggro.
Then here we are again.
I might not have remember everything in the right order but you get the idea. I guess some of that is from new content but not much. Some is also from quality of life changes that might not have been necessary. But most of it, I think, is from the fact that the tanks were never balanced against each other to begin with, and even if we never made new content every bump would have led to the next bump because balancing self-healing against group healing against super aggro against melee DPS against spell DPS against shields against dual wielding against enemy ripostes against resistibility against unresistibility does not work. Especially when everything is ad hoc and just about how things are now and not how they might be later.