DKs were always an incredibly popular tank, even when they were terrible at the start of the expansion, people still played them.
People gravitate towards the strongest tank, which is why bear druids had an explosion in popularity at the start of the expansion. Through a series of nerfs to them, and buffs to other tank classes, you're slowly starting to see people move towards the other tanks. Tank balance at the start of this expansion, pretty much until the end of Nighthold was a complete travesty in my opinion. Right now, aside from a few issues (protection paladins) the balance is in a much better state.
Monks are still incredibly strong and were incredibly strong in NH as well. People were just riding the bear druid wave from the start of the expansion, which is why you never saw a mass exodus. You would have seen more monks in NH if not for bear druids being better, the class as a whole being unpopular, and monks getting a bad wrap at the start of the expansion. There were actually quite a few DHs at the start of the expansion until they all turned into bear druids several weeks in.
Most tanks I know have played tanks since they started playing the game, a lot of them are also stubborn and won't change from the tank spec they have always played. Which is why you see a lot warriors and DKs, even when they aren't that strong. Bears weren't viable for raiding until the middle to late part of TBC (like protection paladins) and druids as a whole were a fairly unpopular class for quite sometime, because nobody wanted to play restoration druid in vanilla, and the other specs simply weren't viable. The shear fact that we have an absurd amount of bear druids in this expansion is testament to how much they buffed the bears to make tanks swap to them. You see a lot of DKs because DKs were popular in WoTLK, and every single one of their specs operated as a tank spec (even though which one was best changed between encounters and patches).