Answer 1: Oh goodie, an intelligent question : D
Well it might be for the fact that Navi and Tatl are simply guardian fairies instead of regular ones. Those two never displayed any magic whatsoever, so it's safe to assume they cannot revive.
Answer 2: Navi was pissed at Link for just shrugging her off as annoying when she repeatedly asked him to listen.
Because that would make the game too easy. I mean think about they are with you for the whole game if they could revive you there is just no challenge left at all.
This is quite obvious: they would then serve a purpose!