why does God hide truth from people until a certain time?
God hides truth because it is His prerogative to keep things secret and hidden until it is time to reveal them. Scripture shows the Twelve understood none of the plain words spoken to them because the saying was hid from them, and mysteries were kept in God's mind until revealed through Paul.
In Luke 18, the Twelve did not understand the plain prediction that He would be delivered to the Gentiles, mocked, spitefully entreated, spat on, scourged, put to death, and rise the third day. Les explains the verse says the saying was hid from them because it is God's prerogative to keep things secret and hidden until it is time to reveal it. Later, in 1 Corinthians 15, Paul reveals a secret, or mystery, which means it had never been revealed before. Les emphasizes that the doctrines and revelations that came through this apostle were for the most part kept secret in the mind of God until He revealed them to Paul.