The short story is that when the first nuclear Enterprise (CVN-65) was first commissioned, Congress balked at the price tag of a nuclear carrier and canceled the entire class then and there. This led to the USN relying on conventional carriers -which are inferior to the Enterprise- and caused all sorts of problems for the USN (and it, unsurprisingly, almost sank the USN's nuclear powerplant program for capital ships and submarines) until the first Nimitz was laid down (which, in terms of design, was simply a modified Enterprise with modern improvements).
... yeah, that was something of a debacle on Congress's feet.