They abandoned the idea of launching Shuttles out of SLC-6 back in 1986
following the Challenger explosion. They realized that the safety margins were just too slim. Instead, the Air Force went to the CELV (Complementary Expendable Launch Vehicle, later renamed the Titan IV) unmanned booster route.
SLC-6 was mothballed for a number of years, then was converted to launch small Lockheen-Martin Athena 1 and Athena 2 boosters. Currently, it is being converted to handle Delta IV boosters.