Why 100% oil filled as opposed to gas/oil?
It all comes down to cost. Gas/Oil shocks are cheap to make because the nitrogen (which is one of the cheapest gases) takes the place of where oil should be (oil costs more).
If you pull one of these shocks apart you will notice there is only just enough to cover the piston and valve, once this small amount of oil gets hot it relies on the gas pressure to stop it aerating. The higher the gas pressure the more chance the shock will not work at all when it gets hot because the pressure forces the valves open.