well oral availability. I think most amino acids pass through the stomach wall/gut. And go into the blood stream directly. I'm not sure how this actually works but i'm thinking that pure amino acids is what the stomach acids break down food into, so then in turn the broken down amino acids are then transported through the stomach either into the blood stream or into the liver and then they will either bind with some receptor or use some transport enzyme to reach their receptor to exhibit their effect. Things like tribulus, and other exotic herbs still must go through digesting because they are not digestible by the human body, they basically are foreign substances that cause something to happen when administered into our blood stream. By using something trans dermal we bypass digestion and ultimately the liver, which detoxes everything that manages to permiate the gut and gets passed through by the digestive system.
If all things were rendered useless by stomach acid like proteins and such then we'd just die.