There are laws of nature involved. But also of men, nations, and war.
Pu-238 isn't found in nature, but has to be synthesized. The precursor material, U-238 is the most abundant form of Uranium, which is convenient. It's also produce via neptunium-237.
The real complication is that the nuclear synthesis processes are also those used for nuclear weapons production, which makes for some serious complications.
Those complications only apply to nations that don't have a permanent seat on the UN security council. There are no treaty obligations preventing the US or Russia from making Pu-238.
Pu-238 isn't found in nature, but has to be synthesized. The precursor material, U-238 is the most abundant form of Uranium, which is convenient. It's also produce via neptunium-237.
The real complication is that the nuclear synthesis processes are also those used for nuclear weapons production, which makes for some serious complications.