Good point mommus and I can only think that it's because it is a relatively new market. The PE cabins started in (I think) the mid-1990s in long haul. Their deployment in medium haul (trans USA distances) is much newer?
Over time, carriers tend to land up with prices that are all +/- what the market will bear. Sounds like Virgin America is doing the traditional capitalist thing - trying to make sure that the price is as high as possible at the start. As they know the price will have to come down over time.