Atmospheric effects. Atmospheric refraction adds significant noise, fully obscuring features smaller than about 0.25 m.
Yes, ground telescopes have ways of cancelling out atmospheric effects, but the required calibration and long exposure is impossible with the orbit of spy satellites.
I don't think it takes long exposure. Ground-based telescopes can make the simplifying assumption that they are looking at static point sources. That makes it easier to correct for camera movement and for optical defects from the lense and the atmosphere.
On the other hand, looking down is a bit easier than looking up because the atmosphere will converge rays looking down, but diverge them looking up. See http://what-if.xkcd.com/32/.
That site also claims that others say Hubble is about as good as the best spy satellites, resolution-wise.
Hubble is of the best spy satellites. It was built by the same team that builds spy satellites, using all the same parts and equipment, tested in the same facilities, etc. It just has a slightly different secondary mirror setup and a very different set of instruments and control mechanisms, since it's meant to point at galaxies and do astronomy, not at the Earth doing surveillance.
Seriously, one of the hardest parts of the Hubble program was figuring out how to do it in the view of the public without revealing the underlying surveillance programme.
Yes, ground telescopes have ways of cancelling out atmospheric effects, but the required calibration and long exposure is impossible with the orbit of spy satellites.