Why is phosphorous acid diprotic and not triprotic?
The two isomers you're referring to both exist and are in equilibrium:



When only one or two protons are removed, resonance stabilizes the anions formed, e.g. the $\ce{HPHO3^-}$ anion:



The trianion (shown below) doesn't have the benefit of resonance stabilization. This makes removing the third proton much harder.