Let me start off by saying I was going to main my bear tank this expac. She was a ton of fun in Wrath and Cata, but their toolkit was gutted in MoP. Instead of Savage Defense proccing a bubble that absorbed physical damage (like a DK Blood Shield), it because dodge. Dodge and avoidance abilities are always considered inferior to mitigation/absorbs when tanking. Your damage intake is unreliable - you could have a great string of dodges or a terrible string of dodges. Bears might have more armor than other tanks, but if they actually do get hit, they get hit a lot more than other tanks do. Their utility is great, yes, but if you are comparing a tank that will reliably survive a 25m heroic encounter week in and week out without deviation to a tank that will mostly survive a 25m heroic encounter (but sometimes gets gibbed hard), which would you take?
Most of the time they are great. It's just not all the time.