That there is the crux of it. Most people who become main tanks are alpha-dogs, thrill-seekers, and highly motivated. This doesn't apply to every MT, of course, but it does apply to most of the tanks I've known in 7 years of EverQuest and however many years of WoW that I've been playing. Some observations:to take on the role of controlling chaos and the responsibility where even small mistakes on my part would inevitably wipe the raid.
If a DPSer is or isn't at the top of their game, it rarely will wipe the raid. If a healer gets disconnected or dies during a fight, the raid can often compensate. If the main tank dies, or a key off-tank dies, it's usually a wipe (Gruul and Void Reaver notwithstanding...)
Main tanks are excitement-seekers. Main Tanks will be the first in their guild to get together a Magister's Terrace group when Sunwell comes out, and then be the first in their guild to run Heroic Magister's Terrace.
Main tanks are dedicated. When there's a drop they want from a heroic zone, they're clearing it every single day until it drops. They're grinding every daily every day to get their SSO rep up to buy that new piece of gear. They clear Kara every week to farm enough badges to keep them at the top of their game. When a Main Tank wants something, he gets it.
Because of all the adrenaline a main tank gets when he loots Moroes after still not seeing the Pocket Watch after 47 Kara runs and counting, the high blood pressure sustained from needing to shield block every single shear or the raid wipes, and the number of badges he had to farm to be the best tank he could be for his guild, he needs his down-time. Also, after 22 runs of Heroic Mech before he got his sun-eater, it would take the pope himself to get him back in that zone.
If his progression has stonewalled, he's going to switch to PvPing, DPSing, or an alt for that new excitement. Tanking old things are boring and the only reason the tank even does these (usually) is because he needs something from it, or someone convinced him to help them out.
This isn't so much a problem that Blizzard can fix, because it's not a problem with the game, it's a personality trait of the people who gravitate to the Main Tank role.
As a MT, whenever people give me grief, I tell them to pug the tank, since I had to pug my DPS and healer for 75% of my badges