Because it’s a literal niche for paladin tanks. You are now hand down the best caster tank in a raid ever. Any boss that does spell damage paladins will be better then warriors or druids on.
It’s a good role for paladins as a tank. You also get to apply everything you have now without having to get hit to run your highest threat damage. Warrior tanks are bad caster tanks because if they aren’t getting hit they don’t get to push revenge their best threat ability by a large margin.
You have an awesome niche. Take the win.
Also you assume you NEED to be crush capped. I’m not at all convinced you will need to be.
It puts paladins as the spell damage tanks.
Druids as the best hard mitigation tanks against things like a hateful strike for example.
And warriors are in the middle as a more midline tank, that doesn’t particularly excel at anything but also isn’t the best at everything.
Tanks have meaningful differences now.