I'm guessing they kept the iPad 2 because it has the same specs as the iPad Mini which they also kept. iPad 3 wouldn't have made sense as it was discontinued. iPad 4 would have been appropriate but no 30pin dock for legacy users.
Also who in the right mind would buy an iPad 2 for 399usd when an iPad Air costs 499usd which is 3 generations newer LOL!
Same with iPad mini vs iPad mini with Retina - 299 vs 399 when the SOC is generations newer.