Why isn’t all non fixed artillery truly mobile?

Frederick the Great of Prussia introduced the idea of mounting the guns and crews on horseback and wagons, the invention of horse artillery. This innovation greatly increased the mobility of field artillery so that commanders could routinely move the guns around and change deployments for maximum effect. At the same time, of course, guns were becoming lighter and the aiming mechanisms more accurate. The result was the introduction of a truly deadly combat arm that would, over time, be responsible for more casualties than any other weapon.
http://www.au.af.mil/au/awc/awcgate/...z/gabr001a.htm

Frederick the Great lived from 1740 to 1786.

So at least in the late era, why are all non fixed guns not completely horse mobile? This foot artillery thing never did sit well with me.

Anyone else?