There's many ways to look at this.
Paladin's threat build-up is quite large and over a general area too, but to compensate their big emergency cooldowns are pretty much once offs which prevents them doing anything using Forbearance a lot or "extra lives" with Ardent Defender unless you want to hit the enrage timer or so.
Warriors are the bread and butter of tanking. They have a median threat build-up, assortment of emergencies, and can AOE tank if needed. But there's a shortfall where they have no predominant "abilities" that would otherwise call them a special kind of tank. It really is a jack-of-all-trades kind of tank, in my opinion. AOE or boss tanking, there's no shortfall yet not dominating force either.
Druid bear tanks have mass amount of health and can take mass amounts of punishment without need for Defence caps or much of the other stats. However their generation of threat lacks when it comes to AOE because all of that is generated in a frontal cone. Plus their threat build-up is scaled directly with their gear - going in a current raid with a lower-powered weapon spells disaster when having amplified damage battles (eg. Twins).
Death Knights lose out with the generation of threat during the rune cooldown periods where everything is generated with white hits, dots and more of the same. Initial generation with Death and Decay is an invaluable thing for them but after that, they have to rely on instinctive rune-power to generate the rest. Not to mention their mitigation is stunted than the rest of the classes. To compensate, they have more emergency buttons with shorter cooldowns.
So yeh, paladin threat is sought after yet there are reasons why raids can choose other tanks that just paladins.
Healing is the source of life. Without it, you're left with death.