Probably an "abudance of caution", they want no overturning of the trial result on appeal. IF he is paralyzed from the chest down, it's not like he' going anywhere. He is confined to a US Army facility, and has armed guards.(and likely some steely eyes nurses and medics too) I don't think either Ft. Hood or Ft. Sam Houston have appropriate facilities and the Army definitely wants him under *their* control, not in some civilian jail.
For some reason I thought he was at a civilian hospital in Corpus Christie????