My understanding is that with X-ray etching/lithography it's significantly more difficult to produce the coherent beam and also to do the focusing of it through a masking plate so you can etch a whole wafer at one time. That's the main reason that they've been working on EUV because they can produce a coherent pulse and focus it more easily. For X-rays you have to turn it into a diffraction grating to make the mask and that ends up more difficult to produce (probably not impossible but much more difficult).