Knight probably knows more about this, but it is my understanding that large bolus doses of whey protein (high in leucine and other BCAAs) have a hyperinsulinemic effect and because they are absorbed so rapidly, a portion of the protein contributes to gluconeogenisis and releases glucose into the bloodstream. For these reasons you want something requires more digestive activity so it is not absorbed as quickly, releasing glucose and elevating insulin levels.
Precisely.. Because it is so damn quick. I'm really not sure about egg protein powder or egg whites as my common practice is to have whole eggs. It would be interesting to know the rate of gluconueogenesis with a large amount of egg whites that one might drink.