Both Athena and Poseidon wanted to be patron deity of Athens. To prove her worthiness for the honor, Athena caused an olive tree to spring up on the citadel of Athens, the Acropolis. Poseidon sought to outdo her by striking the ground with his trident and causing a spring of water to gush forth. But as he was god of the sea, the water was salty. Athena's gift to the Athenians was considered to be more useful, so she became the city's patron deity.
>This is the reason given in the movie so once again the movie has gotten the correct information< :)