Yea, cost. It was supposed to be installed on the Zumwalt, with a guided 155mm cannon as an interim, also with the goal to be cheaper than missiles. Unfortunately both ended up being more expensive than missiles. Replacing the rails isn't something that can be done quickly or cheaply.

It is kinda comparable to hypersonic missiles in that it can penetrate air defense, but that is about the only overlap, the railgun is long range for a gun, but nothing compared to something like a hypersonic missile.