Our Ops Dept asked this very question of Boeing (2008) and the reply was as follows;
"In the RUNAWAY STABILIZER non normal condition, manual trim is always readily available, there is no need to limit the landing flap to flaps 15.
The STABILIZER TRIM INOPERATIVE checklist may be addressing a condition in which the crew must continuously break out the disconnect clutch or in which the stabilizer is physically jammed; it is therefore more appropriate to limit the flight crew to flaps 15 to avoid large trim requirements."
Hope this helps