If the relaxation of the pouch is directly tied to the stress/adrenaline response diverting resources elsewhere, which aids in escape, then it's not so easy to optimize to not loose the pouch. Pouch relaxation wouldn't be the genetic trait being selected for or against, flight response would be.
It can be more harmful for the pouch to be relaxed than not on an isolated level, but if that's counteracted by the increased ability to escape due to resource allocation then it would pass on as a "by product" regardless of its individual effect.
There may not be an existing genetic variation which maintains the pouch control during the flight response without compromise on the degree of speed or intensity dedicated to the escape. Evolution can't select for mutations that don't yet exist in the gene pool.