There are several real reasons why there is a lack of tanking, but a few reasons stand out more than others. Stress is a big one, especially in PUG's. Marking, pulling, aggro switching, making sure the huntard doesn't target the wrong mob, etc. it's a LOT of work, so most of us tanks only tank for people we trust.
A lot of us are losing interest (or rather, have already lost interest) in WoW tanking, because the game seems to be building further and further away from it. PVP, anyone? The single easiest access to high quality gear is PVP. No guild or tedious raid schedule required, only time and perseverance. Hell, you don't even need to win. If you will look carefully though, there is no tank gear, since it is all geared towards dps/healing.
As a last quick note, raiding as a tank is rather dull, even when get shiny upgrades, that let you tank raids better. When dps classes get raid upgrades, not only do they do better raid dps, but more aspects of the game improve for them as well, pvp, soloing, etc.
Maybe i'll return to tanking when they make respecs for tanks and healers free, so i can at least do some BG'ing or solo questing when i want without forking over 50G for 2 hours of fun.