Enriched uranium would work too. But enriched uranium is roughly speaking the ideal fuel for fission reactors, because it has a large ratio of delayed neutrons

Enriched uranium would work too. But enriched uranium is roughly speaking the ideal fuel for fission reactors, because it has a large ratio of delayed neutrons vs prompt neutrons. Plutonium has a much lower ratio (about one third of U-235), so plutonium is inherently harder to control. That's why plutonium is not used directly as fuel, but it's used in a mix with uranium (the so-called MOX = mixed oxide fuels). In a design based on mini-fission bombs, you don't care about control, because control is about not letting the reactivity of the core go too high, while here you want it to be as high as possible.