Several reasons.
1) I don't like tanking dungeons. Or trash groups in raids either, for that matter. I like tanking bosses. Only tanking bosses, however, is not supported.
2) Other people. I like to tank at my own pace. While i'm not "slow", i'm a lot slower than some other tanks i've seen in dungeons. If i don't feel comfortable pulling another group, i shouldn't be pressured into it, or have some dps or healer pull the group, only for them to die because my aoe tools were on cooldown (which was probably the reason i didn't want to pull them in the first place), and then yell at me for not "tanking properly".
3) While i have absolutely no problem keeping up while playing a dps, while tanking, the game often gets a bit too hectic for me nowadays, and i often feel like my toolkit isn't good enough to pull and keep the groups the game throws at me. This probably has more to do with me, seeing as i mained dps classes for the last 10 years, than it has to do with the game being "unfair", but that's just how it is.
4) Tanking feels mechanically different nowadays than it did some years ago. I definitely tanked more during wrath and cata than i do now. I even mained tank during firelands (not out of want, but out of need). I think it has something to do with that whole "active mitigation" thing, of which i think that it works in some cases, but not in all.
5) General rotation and priority system changes in the game. I want to be gcd capped. I want to press a button in every second of a fight. DK tank does not offer that (DK is my main class since day 1 wrath).
I do tank from time to time, but only when i have to because queues take too long. Maybe it isn't even the game at all, but me having to play with randoms since mid MoP. I don't know.