Paladins have too many problems to be considered as tanks:
1. Lack of taunt - this one's a real dealbreaker because even if an encounter doesn't need you to rotate tanks, the moment you lose aggro to a damage dealer, your raid is going to wipe.
2. Shitty threat generation. Their damage is pitifully low and Righteous Fury does not make up for this.
3. They're gimped by really stupid talents like Redoubt and Reckoning, which proc when you are the victim of a critical strike. Hitting the Defense cap in order to become crit immune is a required aspect of raid tanking. This one particularly neuters their ability to hit the Avoidance cap, since they won't be getting a 30% increased block chance at all if they don't receive a crit.
4. Paladin specific gear is optimised for healing, driving the emphasis that they're a healing class first and foremost.
Even if Greater Blessing of Kings spamming still worked, which the consensus of this thread suggests it doesn't, that's still just one of four crippling weaknesses alleviated.
Feral druids may also lack in itemization, crit immunity and avoidance, but unlike Protection Paladins, they have strengths which offset these weaknesses to a significant enough degree that raids may consider them. They deal more damage and threat than Naxx geared Warriors despite their (lack of) damage scaling from gear, they have a much higher health and armour pool which is great for soaking up crits, their BiS damage dealing weapon is a mid level drop from Gnomeregan of all places,