It's the lack of an additional safety system such as Engineered Material Arresting System (EMAS), or crushable concrete. For further information:
Fact Sheet - Engineered Material Arresting System (EMAS)
This is something that's being installed at the end of several runways in the US, but not along its sides. At the very least, it should be installed at airports where there are ravines or similar hazards along the side of the runway.
In this case: If the system had been installed all along the runway, the aircraft might not have ended up in the ravine at all. THAT's the issue, not creating flat area at every airport, which is pretty much impossible.