Why do you think Odysseus chooses to sail toward Scylla rather than Charybdis?

This decision by our hero is yet another example of the hard choices that Odysseus has to make. He is presented with a situation in which either choice will lead to death. However, ever the pragmatist, Odysseus chooses the path that will lead to fewer deaths.
Charybdis and Scylla are two sea monsters that dwell in the Strait of Messina. The waterway is too narrow to avoid them both. One must be confronted. In Book XII, Circe describes each monster to Odysseus. Charybdis' danger is that it creates a violent whirlpool that can destroy entire ships. If Odysseus takes his ship too close to Charybdis, it will destroy his ship, him, and his entire crew. Scylla, while still a terrible monster is less destructive. This six-headed beast is only capable of devouring six sailors at a time. On Circe's advice, Odysseus chooses the root that allows the majority of his crew to survive, at the terrible cost of six of his strongest men.
The choice that our hero makes is by many accounts the right one, despite its gruesome cost. Here, Homer is presenting us with the ethical question of whether it is better to lead a small number of people to their deaths in order to save the majority. In fact, such ethical decisions have been dubbed "caught between Scylla and Charybdis."

This question is very clearly answered in book twelve of the Odyssey. Circe gives Odysseus advice about the perils he will face on his sea voyage as he returns home to Penelope. One of the major hazards she describes is a narrow strait he must traverse guarded by the two monsters Scylla and Charybdis. If Odysseus sails too close to Charybdis, he and his mariners will be sucked into a massive whirlpool, and they will all die. Obviously, that would be a very bad decision.
The other option is to sail on the side guarded by Scylla. Scylla is a monster with six heads. Each of the heads will grab and eat one sailor, meaning that steering toward Scylla will result in the deaths of six men rather than everyone. While this is not a good outcome, having six people die is better than losing his entire crew.
Odysseus refrains from telling the sailors about Scylla in advance in order to ensure that they do not become paralyzed by fear. Instead, they speed through the strait on the side guarded by Scylla.