Why was the mystery kept hidden in God instead of being revealed earlier?
The mystery was kept hidden in God because it was His sovereign prerogative to keep things secret as long as He desired. It had been hid in God from the beginning of the world/age, and God later made it known through Paul, that Gentiles should be fellow heirs and of the same body.
Les explains that the mystery was hidden in God, the sovereign God who created all things by Jesus Christ, and it was His prerogative to keep things secret if He wanted to. God had the Gentiles on His mind, but this was under a different set of circumstances than what was seen earlier. Paul was made a minister according to the gift of God's grace to make all men see the fellowship of the mystery, which from the beginning of the age had been hid in God. Les also notes that one aspect of the mystery is that Israel would be spiritually blinded for almost two thousand years, and another is that Gentiles would come into a privileged position with Israel's God in the form of the body of Christ.
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: