Well OxShoe, if you find a true 1943 copper cent, I will give you a dollar for it and will fly out to pick it up! I really don't know the reason behind only 1 full year of steel, but from a business standpoint, I would think it was supply/demand issue. Once demand for copper went up, the mines kicked into high gear to meet the demand. Likely steel cents were more expensive back then and harder to stamp out than copper due to the hardness of the material.
For whatever reason, I found about 4 1943 steel cents at 2 different parks. If I remember correctly, only 1 was by itself and I was digging everything. Otherwise, I agree with Tony. You have to hope for a wheatie spill.