why does iocasta dismiss the oracle's prophey

Jocasta does not believe in the divine prophecy: she doubts the word of any oracle. She believes there is no such thing as fate rather that the future is based on chance and coincidence.

She dismisses the oracle's prophecy beacause of the previous prophecy which she sees as unfulfilled. It was prophesied to her and Laius that their child would kill his father and marry his mother. She thinks this child is dead and the prophecy therefore unfulfilled; she thinks that she has successfully changed her fate by her own actions, hence she believes that the prophecy of these gods is not to be relied upon and one can always have a hand in how their life turns out.

Of course this was all before she discovered that the inital prophecy was indeed fulfilled.