U-235 is a finite resource. Sure, we have technologies like fuel reprocessing breeder reactors, thorium reactors and heavy-water-moderated reactors, but they've not been perfected or widely implemented. By some estimates we only have about 150 years left of economically extractable uranium at current rates of extraction. If breeder reactors and widespread reprocessing were never developed in the Fallout Universe, combined with the vastly increased use of nuclear power, it's quite possible that the world is running low on uranium by 2077.
One of the main reasons we're not using breeder reactors and thorium today is that light water reactors are better for producing plutonium for atomic bombs. Most of the early research into nuclear power was done by the military for this purpose. Nuclear fuel is cheap compared to the plant, so there's no real reason for companies to spend trillions on R&D for new reactor classes for a saving on fuel, which is only a fraction of the overall cost. (A similar thing has held back renewables. Most of the research into thermal powerplants, boilers and steam turbines was carried out initially by the Royal Navy, Kriegsmarine etc. for use in ships. One of the reasons that renewables have historically been so expensive is that they have to absorb the R&D cost into the cost of the power they generate, whereas thermal power plants do not)