I think she knows he's the one (doesn't she or someone else say that later in 2 or 3...?).
In my opinion she doesn't tell him because at that moment he cannot possibly believe such a thing, he's not ready to hear it, he doesn't feel anything of his own power. If she told him flat out "yeah dude, you're the One" my guess is he would be sooo scared of his responsibility that he'd get all paralysed.
Maybe he wouldn't decide to save Morpheus either, cause the life of the One is so precious?
That way on the other hand, he thinks he's not the One, since the great oracle said so. So he's much more courageous and decides to sacrifice himself to save Morpheus. Thus he ends up doing exactly what he had to do, discovering his oneness in the process
My interpretation, but I'm not a Matrix specialist, I barely remember them.
