The reason is how tanks are designed currently, nothing challenges you as a tank other then leveling and pulling dungeons outside of that bosses in EXs and some Savages either don't move or auto position themselves and outside of positioning and living we only do DPS which our DPS isn't rewarding this expansion. I can do 10k DPS as a DRK and a co-DPS can do 14k but their DPS makes it or break it mine doesn't matter in the grand scheme of things, not only that but every tank has way too many CDs for any situation where you won't die whatsoever even with 1 measly tank CD.

This expansion it's great they balanced tank DPS with getting rid of things like slashing resistance, making them more accessible but it came at a cost, as it stands tanking is the least rewarding role when played right you're a downgraded DPS that can survive a beating while hitting the boss with a foam bat. I'm hoping 6.0 changes it but i'm not very hopeful the only reason I haven't quit raiding is because my static needs me as a tank and if I swapped to a DPS it'd just make things inconvenient since I've been a tank main since I started the game.

That's just my 2 cents.