It is probably also a lot easier for the US to launch a space probe with a nuke power source than Europe.
Regulatory requirements are very different and I am sure the US would not be too happy if Europe where to launch something with a nuke onboard without their approval.
It's not "a nuke", it's a relatively small amount of radioactive material that is sealed inside a very strong capsule designed to survive almost any imaginable catastrophe during launch. The launches are from the coast out over the water. Yes there are still some risks, but in the grand scheme of things, the risks are nothing compared to the importance of exploring the universe.
Regulatory requirements are very different and I am sure the US would not be too happy if Europe where to launch something with a nuke onboard without their approval.