Haste also increases your autoattack speed, which is far more important than what it does to energy regen.
More attacks = more Gift of the Ox and Elusive Brew stacks. Increased expertise is the best way to do that, which haste coming in second. As a bonus with expertise, you also get fewer misses on Keg Smash (which is always a loss of 1 chi, 8 energy, and 1 GCD) or Jab, which leads to better uptime on your Shuffle in the long run.
Agility is technically better (it increases crit, dodge, and attack power, which increases the size of your heals and shields), but it's usually not twice as good as the secondary stats, and gems and the like tend to give you 2:1 for secondary
rimary stats in MoP. That's why gemming expertise and haste tends to be better than gemming agi, even if agi is technically a better stat.
It's worth noting that currently no strategy for gearing is going to get you to a point where you're taking comparable damage to any other tank, so you have to rely on your self-healing and avoidance. That will probably change post-release, since it's going to lead to a very RNG-centric tank with no significant health buffer. But for now, this is the best we can do.