I thought it was odd that the iPad 2 was kept around for sale. Why not do the iPad 4 as the older model? The new Air is a big enough step forward, especially over that even thicker and heavier model.
Then I thought of a reason: Lightning. The iPad 2 is the model that still had the old style 30 pin connector, which is used for a variety of devices like Point of Sale terminals. And who knows what industrial uses it might have.
Rather than bring back the iPad 3 with the old connector (which would be weird since it's been off the market for a year) keeping the iPad 2 makes a little more sense. And give its older technology, the profit margins on that one must be huge.