I think the main issue is just that true role isolation comes with a price. That, and the idea that good tank = knows dps; good dps threat management = knows tank. A good tank knows the threat periods for each dps class, or else (over)compensates enough, to never be pulled from unless heavily outgeared or a dps starts a new mob with a Firestarter crit, etc. But a bad dps, for instance, will not realize that pulling a Heavy Thrust at the same time as his MRD (no Defiance) tank's Heavy Swing, will pull, if only for a second, or that Bloodletter and Straight Shot in the same GCD is a 290 potency that the tank can't cover until his second move in the enmity combo, when it can be adjusted for by as little as using it a GCD later. Often as the number of really good tanks increases, so too does the number of dps unconscious to threat mechanics. Usually the dungeon grind is enough to gradually figure out how that works for first-time dps, but many simply don't seem to think of it as an issue of learning. That's one of the critical problems--personality, in a sense--along with the simple fact that one often never really *has* to get threat-work down as dps even during the dungeon leveling experience. (And not at all anywhere else.)
That said, I've done more Stone Vigil's with DRG or Bard tanks than PLDs or Warriors, sometimes with better results even. It just takes coordination. When tanks get bored of the job, its usually because they've given up on holding the subtle maximum of what they can do by playing according to their party, which in some compositions is more challenging than maximizing dps, or they never realized that that was an issue to begin with. Threat isolation (one dps, usually bard holding/kiting solely melee enemies, being untouched by the healer, so that the heals on the other dps, likely tanking ranged, don't pull at all), Binds, Sleeps, etc. a lot of that doesn't even figure into some player's minds until they're forced to use it because some tank writes "sry, dinner" and leaves at the start of a dungeon.
Generally, healers and tanks must both know enemy mob's dynamics. DPS just need to know tank dynamics to an extent and only when much more geared, and kill stuff fast [fast enough to oblige kill order or eliminate abilities by killing their users, in some cases]. DPS are therefore more internally complex to compensate for what little they need to watch in the battlefield.
(That's not to say that tanks couldn't afford to become more complex in their own right--it would just likely mean that eventually dps would have to re-enter the issue of threat management and learn how to deal with it, likely while yelling nonstop at their tanks for doing the best their class is capable of...)
It's kind of a tough issue... Perfect tanking can be interesting for a while, but as long as there's not much benefit in the fine edge of it, or in maximizing your own damage while still properly holding threat, the tank role is fairly dull simply because the classes are somewhat dull. But if we were to get more tanks with more interesting internal mechanics to worry about, it's hard to say if they could perform that job correctly, while already faced with a sort of a dps-clan that are well-rutted in a certain unnegotiable idea of what dps itself should be like. Maybe if dps were given more ability to snap-tank or, better yet, assist (rather than just make up for) the tank within the interplay of positioning and mitigation, that bridge would be a bit easier to cross, but it's still pretty vague to me.
tldr: During the leveling process, I like tanking for the most part because people can still pull from me if I don't watch my enmity. I liked being an undergeared tank in AK with AF+1/Zenith dps. I like tanking as a warrior when I can go full out on dps or really need to hit each mechanic of the fight correctly to survive, but in other circumstances I'd really just rather play melee dps and try to perfect my rotation while dodging nonstop. If it's a DF, I'd rather go healer and dps because that's the position I can put the most variance in compared to most healers I see. I think that's the case for a lot of would-be tanks. It's just not quite interesting enough if someone else can do it well.
Edit: (to above) I'm damn glad we can still be one-shotted by certain attacks if we've no CDs up. 90% of health or so would be fine as well, but anything less than that would make tanking rather dull. I don't mind if a healer has to make up for it in preparation and reaction, but between healer and tank someone really should have to know the fight, not just meatshield it. I just wouldn't like it if anything short of Twintania-(relative)level monsters could one shot melee with their CDs up (i.e. DRG with Keen Flurry and Foresight), partly because I would like to see dps become more tank-capable situationally (not as a replacement for real tanks without 'real' and optional changes).