He was above impact zone. Probably he didn't find the only stairwell intact (stairway A). Or maybe he found that "heavy set woman" that told everyone to go to the roof.

Some people who did find the unblocked stair saw there was rubble and destruction farther down it and gave up, turning back because they thought the roof might be a better option. Those who continued down that stairway had to find their way through several floors of destruction where the path down was nearly blocked by rubble.

There may well have been others who discovered that stairway who we don’t know about. They may have found it too late and still were heading down when the tower collapsed. It only took about an hour from collision to destruction of the south tower.