What are the reasons for this do you think?
A bridge has failure consequences that are predictable: bridge breaks, people on the bridge die, local economy suffers. All of those are understandable by an elementary school student playing with toys and imagining the town around it.
The failure consequences of fission plants were not known to include things that had to be discovered years after Three Mile Island, Chernobyl and Fukushima. Not to mention -- who knew that exhausting hot non-radioactive water into a river could be bad for it?
The failure consequences of Thalidomide were not known for years. The failure consequences of dioxin usage were not known for years: https://en.wikipedia.org/wiki/Persistent_organic_pollutant
Let's also include the US propensity to fall victim to charlatanry: enough of us want to believe so much, we will give lots of money to Juicero, uBeam, and Nigerian Princes. (Hyperloop, cough.)
So if you want to do something big, it needs to be well-understood. If you want to do something new, it needs to be proven and well-examined.
Assuming we did, to get away from mechanical turbines and generators as the heat->motion->electricity step would require a https://en.wikipedia.org/wiki/Magnetohydrodynamic_generator -- which have had more success but still face serious material issues.
As for "why does it take so long?" -- the first of anything takes longer. And the US has terrible problems in building anything new or big.