There are two very different reasons Odysseus didn't tell his men about the risks of the voyage. First and most simply, he didn't know most of them. He didn't know how magical people like Circe would be, he didn't know about the Cyclops, etc. (Some of the risk he generated himself, too, like bragging to the Cyclops.) Second, it's not in his character. Odysseus is the great deceiver and manipulator. He's always improvising, and he primarily tells the truth when it best serves his purpose. Other times, a lie or an omission will do just as well, and that's definitely the case here.