Bears share a lot in common with warriors so far as tanking goes. You can do a side by side comparison of abilities and come up with a one to one relationship with nearly all the abilities. There are minor differences here and there; bears have fewer abilities that in general do more to make up for it, but the overall feel is about the same. Both bears and warriors are highly mobile tanks reliant on rage. Put plainly, we charge around a lot and have anger management issues.
This is in direct contrast with paladins and death knights that have a more stationary approach to tanking, they prefer their targets to come to them. The price they pay for the tools to do that is neither can charge. Both camps can get the job done so the decision is more about what type of player you are. If you prefer melee over ranged you are going to like warriors and druids, if you prefer ranged you are going to lean toward paladin and death knight. What, all tanks are melee?
It all comes down to how you are used to thinking; druids, warriors and melee in general are thinking at the end of a pull, "Where am I going next?". Paladins, death knights and ranged folks are going to be looking around at the end of a pull asking themselves, "What can I hit next?" The difference is subtle and at times non existent, there is nothing stopping a druid or warrior from pulling something at range in the same sense that there is nothing stopping a paladin or death knight from running in, but the class mechanics do favor one tactic over the other. So the first step toward becoming a bear is wanting to eat, drink and breath melee.
If melee isn't your thing then go play with death knight or paladin, probably paladin as you can heal rather than melee as a second spec. Having tanked on both I'd say the paladin was slightly easier to manage, but it is so close you should just pick the class you like the flavor of better. They are pretty much opposites when it comes to undead disease carrying monster vs light wielding holy warrior.
Back to bears; now the only choice left is between a druid and a warrior. This choice is defined mostly by whether or not you are a good fit for a druid. Druids are hybrids in the truest sense of the word. If you are only interesting in tanking and want to specialize to be just a tank then warrior is the way to go. If you want to mix some DPS in with your tanking and perhaps even moonlight as a healer then druid is where it is at. Basically people who can't or don't want to make up their minds prefer druids so they don't ever have too.
What really sets bears apart from other tanks isn't how well they tank or how they do it compared to others, it is what they are doing when they aren't tanking. Many fights only require one tank, some fights only require one tank for one part and two for the rest. Feral druids fill these needs fluidly by having the ability to go cat and for all intensive purposes become a DPS. Even without the benefit of DPS gear or a hybrid bear/cat spec a bear tank can bring it in cat form. No other tanks can come close without changing specs.
The opposite is also true and quite helpful in 5 and 10 mans. A feral druid doing DPS can tank quite effectively in an emergency if specced for it. I cannot count the number of times I have tipped the balance from wipe to win just because I was able to tank for a few crucial seconds. I also can't count the number of wipes I've caused, but that has more to do with my reckless nature and less to do with ability. Lets just say I consider tanking talents to be DPS utility talents. ;)
So the second reason to consider being a bear tank is if you favor flexibility over functionality. Warriors have a few really neat tricks up their sleeves, but can't flip roles mid fight. At the end of the day warriors get as much amusement from their bag of tricks as druids do from their shifting so it comes down to which appeals more to the person making the decision.