One, it may not be practical at orbital reentry velocities. Most military supply drops come from large cargo planes at much lower altitudes and subsonic speeds.
Two, it may not be economically practical for NASA. The military's mission requires it to spend whatever blood or treasure it takes to get the job done. If a battle can be won, or a war can be shortened, by using a hideously expensive delivery system, then it makes sense to use a hideously expensive delivery system. NASA, having to make a totally different set of tradeoffs, may have found that their non-military objectives can be met more cheaply by other means.
Three, JPADS may not actually offer a significant mass savings. What costs you more in lost payload? An empty glider reentry vehicle, or a JPADs reentry system? Especially after you've beefed it up to the point where it can handle atmospheric reentry?
I mean, JPADS is a cargo bay, a steerable airfoil, and a guidance computer. And the X-37 is a cargo bay, a steerable airfoil, and a guidance computer--that can land from orbit.