I think also as the range of stuff the Mac does continues to contract (less software as legacy was dumped for 32 bit, no boot camp, shrinking gaming) the applications that need significant power continue to shrink.

There's really not much that still runs on a Mac that needs more than the base Air. Some more RAM would be nice, but unless you're a high end video editor the use case of higher powered Macs for the price differential seems pretty small.