Carbon from bicarbonate is used for transformation of acetyl-CoA into malonyl-CoA. Bicarbonate gives only one carbon, and if it would give two (both) carbons, it would be lost. On the other hand, two carbons are necessary for the incorporation into the new fatty acid. So, the reason will be that bicarbonate cannot give an adequate amount of carbons, so it cannot be incorporated into the new fatty acid.
