what does the B-57 platform do that a modified Drone can not?
Cost little to acquire and operate.
Fly high(ish) with plenty of space and weight for custom instruments, while needing neither a satellite link like the Global Hawk nor incurring the high operating cost of their U2, which needs its hours saving for doing what it can do uniquely.
I don't think the air above Kandahar is of any more interest to NASA than the air above anywhere else, notwithstanding the cheeky U designation/civilian operation of the U2 from the 1950s.