After diffusion you get a mixture of the isotopes, which is useless by itself. You have then have to separate these very similar atoms by atomic weight, and for that you need centrifuges, because the only difference between those isotopes is mass (which causes diffences in a flight path in a curve, similarily to how GC/MS works), which means that any other separation method will not enrich the isotope you are looking for; U-235 and U-238 diffuse identically, or at least so very much identically that you can't make an industrial process out of the difference.