Uranium is a mineral and can be purchased from many places and stored then used as needed.

15 countries mining it currently: https://en.wikipedia.org/wiki/List_of_countries_by_uranium_p...

There are also more countries with estimated reserves: https://en.wikipedia.org/wiki/List_of_countries_by_uranium_r...

While an undersea cable requires continuous operation and the agreement from a single country where it starts. If they suddenly decide for whatever reason to turn it off (say they're short of electricity) then the supply is immediately stopped.