It's because of the patent protection process.
Instead of the way it was originally intended, today's patent protection process primarily serves the drug companies at the expense of the American people.
For Big Pharma, it’s a hell of a lot simpler to either buy the rights to drugs developed by others and then raise the prices many times over, as with Sovaldi, or to obtain a medication already in existence and then, using monopolistic control, raise the price as much as 500% or more, as in the case of the EpiPen.