My guess is it's a transitional period. The new low-end 13″ MacBook Pro, the new 12″ MacBook, and the outdated 11″ and 13″ MacBook Air probably ought not to coexist. But the MacBook Air is important to Apple as their entry-level (i.e. cheapest) laptop, and the MacBook is still too expensive and not quite featureful enough to replace it.

I think Apple would like to drop the MacBook Air in favour of a cheaper MacBook at some point, but they can't right now.