I did some digging in my Zalogas, and it seems the main reason why none of the M10s from Tunesia were used in Sicily was a heated argument about the usefulness of self-propelled TDs.
A number of influencial Generals, including Patton and Bradley were not impressed by the performance of TDs in North Africa and the advocats of artillery, led by General McNair, head of Army Ground Forces and an artilleryman himself, firmly believed in AT guns, dismissing the advantages of self-propelled TDs. The general(s') opinion was that the best way to deal with enemy tanks was to use tanks on the offense and ATGs on the defense.
With the debate going back and forth in 1943, it seems obvious why even the existing TD units were treated with disdain in Sicily.
Now if only you could convince them to include Salerno in the base game, we'd both be happy.