A single centrifuge isn't very useful, they are best used with many in a line. Remember, they are trying to separate two isotopes by a difference of the mass of 3 neutrons, that's a very precise and slow process unless you're doing it in bulk.
It requires a lot of specific materials and a ton of energy to do. So that makes building and maintaining secret enrichment facilities challenging.
Distributed would be incredibly slow and not worthwhile, you'd be spending more time moving the material than enriching it, especially given the number of stages you'd want to do it quickly.
