The difficulty doesn't need to increase at a high rate for a miner to be unprofitable.  The miner just needs to cost too much up front relative to how much it will earn.  If all the manufacturers cut their pricing in 1/2 then pretty much every miner (except BFL Mondarch) would ROI.  The problem is that as miner gets close to being a good buy tons of people jump on it and drive said difficulty higher.  Vicious circle.