Okay so I had a big long post ready to go, but uh... I figured it out. (A long story that essentially boils down to "Don't manually install mods or bad things happen")
Here's a better question: Why does MKS still list ground construction as a dependency in CKAN?
Ground construction is bundled with the github download, being a dependency and all. It's also labeled as a dependency on CKAN... which is fine, because normally it would automatically grab it. Except that ground construction on CKAN is only listed as being compatible with 1.4.5, although we know otherwise. Of course, MKS itself says it's compatible with 1.6.9, so why would you think to check otherwise? This leads to the counter-intuitive situation where you cannot ask CKAN to install MKS unless you specifically dig through its dependencies, notice that ground construction asks for 1.4.5, and then manually tell CKAN that 1.4.5 mods are okay (just for this one).
I guess this is technically just a CKAN UI problem, but I don't see why the ground construction dependency can't just be shoved into the CKAN repository to bypass this problem. Assuming that the mod author of ground construction isn't already working on resolving it, of course.