Mostly the fact that
1. as someone mentioned already, x-ray lasers are not easy to generate, with designs sometimes using nukes to pump them [see https://en.wikipedia.org/wiki/Project_Excalibur ];
2. while x-rays can be reflected, x-ray mirrors don't behave like optical mirrors because the wavelength of an x-ray is in the same ballpark as the size of atoms (10 nm to 0.1 nm, with atomic radii being in the 0.1-0.01 nm range), and they only reflect 100% of incident radiation when it hits them at very shallow angles ("For gold at 1 keV, the critical reflection angle is 2.4 degrees") [see https://physics.stackexchange.com/questions/128247/how-would... http://henke.lbl.gov/optical_constants/ https://en.wikipedia.org/wiki/X-ray_optics ].
When talking about lasers pumping out enough energy to be usable as weapons over distances of hundreds of kilometers - to bounce them off a satellite - you will need a mirror that can reflect that ray almost perfectly, or you'll just blast your own satellite out of the sky. And to do that, that mirror will only be able to reflect your ray at a shallow angle, which pretty much rules out bouncing an X-ray off it (unless your target is in orbit or, I suppose, if you have a fleet of such satellites, and coordinated enough to bounce the ray down 2° at a time).
[Also see http://www.projectrho.com/public_html/rocket/spacegunconvent... , C-s for "X-ray" and "grazing", but that whole website is a goldmine]
Also, I said "pumped x-ray laser" for a reason; this type of design usually involves blowing up the energy source and/or the laser medium. Fits the missile or shell use cases.
> The whole point of ordinance is its cheap
Not in the US, with how much missiles cost.
As for backscatter, that's only an argument for a shell instead of a ground installation - you pulse the laser high up, where the backscatter is of no concern to you.
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[0] - https://en.wikipedia.org/wiki/Mean_free_path#/media/File:Pho...
