It's because the traditional aerospace contractors of red-state areas (Ares was corporate welfare for them) stick to it.
This is why the Mars Climate Orbiter crashed. JPL/NASA scientists use metric of course. The first of the recent successful return-to-Mars systems was Pathfinder, all done by JPL. Worked great. Next one, Congress forced use of outside MIC contractors for some part. They used imperial units, and had a fresh-college hire working on the guidance computations. They passed information back and forth on plain files.