Why building a stopway instead of just building a longer runway? I don't get the point of building a stopway. If you have the space to build a stopway, you could just extend the length of the runway.

Why are stopways necessary?
Overrun areas are almost always filled with some form of loose material... sand, for example... that will slow a plane down with extra drag on the landing gear. The idea is that if a plane has reached the end of the runway, it's already in trouble and is probably having problems braking.