Why does Odysseus choose only to tell his men they must deal with the sirens and does not tell them they must also face the Charybdis and Scylla?

Odysseus chooses not to disclose the full danger of facing Charybdis and Scylla to his men because he wants to prevent them from succumbing to fear and despair. By only informing them about the Sirens, he maintains their morale and focus on the immediate challenge, allowing them to concentrate on navigating past the enchanting songs of the Sirens. Revealing the additional dangers would likely overwhelm them, jeopardizing their ability to face the trials ahead. Ultimately, Odysseus seeks to protect his crew's resolve and ensure their survival during the perilous journey.