Teh problem is that tank has too much responsibility involved in his role. Tank has to lead people, tank has to organise pulls, tank has too much personal responsibility in, well, everything.
Me personal loved tanking in BC, kinda loved tanking in WoTLK, never tanked in Cata... and kinda hated tanking in MoP. When I started playing, threat management and cooldown usage were two general skills that differentiated a good tank from a bad one. Also, tanks had to collect two gear sets, avoidance one and mitigation one, because avoidance really mattered in some fights. Nowadays every random Jonh and his dog can easily manage threat, and tanks wear common damage dealer's gear. Without threat management, for me a tank's gameplay is nonexistant, and having a possibility to top meters both in damage dealt and healing done only makes it worth, because not only you have to receive tons of damage to boost your Veng, which is absurd due to main idea of a tank is avoiding a damage, but you also make your fellow raiders look bad (lol his damage is worse that a tank's one, lol).
back to a question: yes, I believe, there is way less tanks nowadays. Though I think it is not because of changes to a tanking system, its just people don't want to take responsibility of a tank for a bunch of randoms. Easy respeccing and dps gear also take their toll here.