I hate to give my age away, but I was on the staff at Randolph when the choice of the T-6 was made (I even flew the PC-9 when it came around on the demos). We didn't want it, the Navy forced it on us as they liked single engine turbo props and it didn't approach the performance of their T-45s, which the jets we liked did. Anyway, a decision made at the DOD level for JSUPT.
The USAF wanted a twin engine jet, several were available but........
Ferd