Depending on the alloy, magnesium may be too brittle and not ductile enough for use in a car body (bad in a wreck). The metal is expensive and can catch fire. Magnesium is subject to corrosion from salt on the road. Magnesium may be too difficult to machine (form) into body parts. Many magnesium parts (aircraft engine gearbox housings, etc.) are cast and relatively thick but still light.
