The Enterprise has eight reactors, a triple hull and armored plated stern. It came with all the options. The story is that Admiral Rickover did not want a nuclear powered carrier. So he made sure it was really, really expensive. So expensive that CV-66 and CV-67 were conventional.
As a side note, at full speed the shaft from #3 engineroom has so much torque on it that it is twisted one and one half turns from the gears to the prop.
I'd say AMERICA and Kennedy were conventional for several reasons. One could have been projected availability of trained reactor operators. You can train a conventional Snipe pretty quick. Most are standing second and third level watches before a Nuke ever sees a ship much less qualifies on it.
Another thing would be cost as we had the WW2 & Korea war debt. America was awarded under Ike's term and the keel laid right before JFK was sworn in. Coventionals were cheaper to build, easier and quicker to train the Snipes, and DFM was fairly cheap.
That said I remember seeing some piping blueprints in Engineering Central when I was tracing down some piping that had CVN {slash} CVA-66 and nobody knew why.
BTW some carriers had customizing done. The AMERICA was the only Sonar Dome carier.