I have to assume fuel costs are a big part of it. In Jan 2009, I booked a round-trip to HNL from DCA for $400 on United. The SFO-HNL flights were overbooked all day so they desperately needed volunteers. I got $600 in travel credits and an overnight in SFO so I made money on the deal!
Miss those days...now it's difficult to even get to the West Coast for under $500.