Just a wild guess...
I would think that the engineers would have thought at the time, that you are most vulnerable to stall the plane with the initial climb when also having slats extended for best rate of climb. I know on the B727 even 2° extended slats makes a big difference with the climb. Maybe they thought that you wouldn't ever stall at cruising Alt / speed, thus only have them fully deploying if they were already deployed. You wouldn't want the slats to deploy from a clean wing confg on a high speed stall for instance...
Im no expert on the MD-80 but in my opinion this makes sense???