Why is amide protonated on oxygen rather then in nitrogen ??

Consider structures, ##RC(=O)NH(R)## versus ##RC(-O^-)=N^+HR##

The second structure is the amide equivalent of an enolate anion. Evidently, more electron is concentrated on the oxygen rather than the nitrogen centre, hence the oxygen is more basic. Even though the nitrogen has a formal lone pair, it tends to donate electron density to the oxygen centre.