Lots of reasons, I used to be a tank main but pruning and the dumbing down the tank kits turned me off to the classes.
Most of the tanks lack an interesting gameplay loop. BrM prolly has the most interesting but its still pretty meh.
Tank balance is pretty much a joke atm which only makes point number 1 even worse. Feeling pidgeon holed into prot warrior (old main before Legion) just sucks.
To be clear on the last point, no I dont think Prot warrior should be nerfed. The other tanks should be brought up to their level. Its incredibly boring to look over and damage meters and see you are 80% behind your damage dealers sometimes. It just feels incredibly unrewarding.
Note: these are my reasons for not bothering tanking.