Like any company, Facebook sells a product to its customers. The product is user data, and the customers are those willing to pay for access to that data. As a result, Facebook is under constant economic pressure to maximize its ability to sell that data to its customers.

Zuckerberg has a huge incentive to promote the idea that privacy is passe or somehow doesn't matter, because the more he can convince his users to accept having their data made public, the more value he can deliver to his customers.