Why they're not on the geostationary orbit?
It's because the geostationary orbit - and indeed, there's just one such orbit, a one-dimensional curve in space - only exists above the equator while the GPS has to cover the whole planetary surface, including the points closer to the poles.
However, it's untrue that the GPS satellites are located at the LEO, either. Low Earth Orbit is defined as 160-2000 kilometers of altitude. However, the GPS satellite constellation is located roughly 20,200 km above the surface - over one half of the geostationary radius - in such a way that the position of each satellite returns to the same place twice per 24 hours. This is very convenient for synchronization and planning.
The Galileo satellites will be at altitude 23,222 km. It's also an intermediate circular orbit, much like for the 19,100 altitude of the GLONASS whose orbital period is about 11 hours.