Ground Clearance
I read many months ago (so I can't remember the source) that one of the issues for the AT-6 was ground-clearance of weapons. The wing is relatively low (short struts), and it gave limited ground clearance for some of the weapons they planned to hang on it.
It may have also been a problem for the jammers (Air Force slang term for the self-propelled weapons-loading-tugs) to get under the wings. And I think they would want universal support equipment (ie, a jammer that worked on anything) to minimize the number of C-17 flights it takes to get the Ground Troops set-up in a combat location.