Geostationary orbit wouldn't work for a GNSS: all satellites would be in the same plane, so it would be impossible to distinguish between the northern and southern hemisphere. This would render it inconvenient in most countries and unusable anywhere near the equator.
I thought the same, but they're actually below geostationary orbit. They have 12 hour orbits. Also, they don't orbit the equator, which is required for a geostationary orbit. See my comment above with more details.
I think this article, that most here have probably read or heard about considering how popular it was on HN, explains why quite well! It is a very long article though (which is awesome, imo!). The whole thing is an incredible primer on GPS .