I think the changes to healing are the leading reason why bears are less common.
I think Blizzard did deliver on their promise to make healing in Cataclysm more challenging, enjoyable, fun, or whatever the adjective of the day might be. There is however a very direct link between changes in healing and changes in tanking. Because Cataclysm healing is more challenging, healing in a different context can be more or less difficult. It's easier to heal a shield tank simply because of that consistent and very predictable damage reduction. Healing a bear tank however is significantly more random because they have higher avoidance so often take no damage and the shield can either absorb a significant amount of the hit or none at all leading to a lot more variability in how much damage a single hit does. Because of this less predictable nature of incoming damage it is more difficult to heal. Not because there is necessarily more damage taken but more because it is more difficult to decide what type of heal to use because it is harder to predict the next hit.
I think Blizzard did an excellent job on redesigning healing in Cataclysm. However they need to invest more on the tanking half of that relationship because rocking the boat on one side will naturally cause waves on the other.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.