Because streamers can’t handle losing to tank specs for some reason. 
The reality is that tanks don’t upset the flow of gameplay any more than healers do. People just have this oddly irrational disdain for tank specs. Some try to claim it’s because they’re useless while others will claim because they’re all powerful.
Ultimately, Blizz will cater to the streamers and break out the nerf bat the second a tank finds any degree of success in PVP, even though they claim they want the meta to “evolve organically” when it’s Rogues, Mages, and Locks ruling for literally several expansions on a row.