My guess is that the derivation of energy or de novo lipogenesis from protein is somewhat limited by how fast the liver can convert the protein to glucose via gluconeogenesis. Maybe some protein will be excreted if unused. Fat requires very little effor to be store, however. Glucose is in between. The reason glucose has such a bad reputation is that it releases insulin, the storage hormone. Insulin hastens the storage of fat in adipose. It could also hasten de novo lipogenesis, I guess. This is all conjecture, though.