It might be different now, but pre cataclysm the reason most boxers chose a paladin for tanking was the ease of AOE agro. You could and still can make a single macro for a paladin that will rotate all of their spells on each button. This made it easy for boxers to focus on the healing and dps buttons for the rest of the group. So it didn't matter what you were spamming, the tank was always wailing away doing their job.

You don't have to manage a paladin tank. They are on auto pilot.

Other minor reasons are paladin tanks are front loaded on agro because they can mana dump on a pull. Warriors were back end loaded on agro. They had to charge in, build up rage etc before they got going.