I believe the reason for the reduction in tanks and healers is due to the content that is used to measure the levels of tanks and healers in the game, and also what people want from this expansion and how they go about playing the content toward the twilight months of an expansion.
Toward the end of an expansion I find that many people tend to just want an easier time getting through SoO for the Nth time, and trying to achieve that as a tank or healer is just not going to happen. You can take it easy as a DPS as there isn't as much demand on giving it your all and paying full attention during a run, so the tank and healing role slowly starts to see a decline in how many people play it as people increasingly feel the need to take it easy. Every individual has a point where they think "meh, cba to tank this time"
We also look at dungeon content when we see how many tanks and healers are currently playing, which is where you would want things to just be over and done with so you can finally get that item you've been after for months. So why do it as a tank or a healer if you want to take the easier and less stressful route? It does involve people being shitty to tanks and healers, but I believe that playing the roles themselves is also a bit more stressful for the person involved, so being a tank or a healer is the first thing to go when someone has reached a point in how much effort they can bring themselves to put in to the game; some line that, when crossed, sees them queue as dps.
I would imagine that looking at a graph showing tank and healer population over the course of an expansion would show some peaks and valleys that correspond to major patches. I, at least, think that this would be the case. It's not as simple as people just quitting because people are arseholes to tanks and healers in particular, but it is something that contributes to the erosion over time of people's desire to make an effort.