Because they promised to cut prices by an order of magnitude. Following a promise like that, you can't just offer a 10-15% price reduction.
Plus, if they offered at twice the current price, I don't believe they would be competitive. Part of the reason they offer a low price is that they have still-new, not-fully-tested technology. Clients accept the increased risk, but they expect significant savings in return.
Also, for certain flight configurations, SpaceX's pricing isn't particularly competitive even at its current level.