Gradual Revelation
Q. Why should revelation happen over a period of time and not all at once?
A. Thanks for your question. The answer to your question, as I see it, is because people need time to transition. We see this all over the place. When God commanded Abraham to bind Isaac, He eased him into it by saying, "your son, your only child, whom you love..." (Sanhedrin 89b, cited by Rashi on Genesis 22:2). God caught Moses' attention with the burning bush. He called to the prophet Samuel in a familiar voice. In all of these cases (and more), He could have appeared more abruptly, but He took the gradual path as a concession to human nature.
This is also the case on a national level. Those over 20 years old who left Egypt passed away in the wilderness and it was their children who inherited the land of Israel because a nation enslaved for hundreds of years would not transition well to conquering a land.
Along similar lines, Maimonides explains that many of the commandments serve as a "halfway house" between the way things always have been and the way things optimally should be. For example, the servitude permitted by the Torah is worlds different from the slavery that was ubiquitous in the world at that time. But why should it have been permitted in any form at all? Because going from all to nothing is a recipe for failure. People need to transition.
Another example: animal sacrifice was permitted, but only in the Temple. This still allowed, but seriously reduced, a familiar form of worship. To abruptly put an end to an ingrained practice would be very difficult for the people to follow.
So if revelation came all at once, it would be a very abrupt shock to the system, turning people's lives upside-down all at once. If you want people to be successful in change, introducing it gradually is a much better strategy. God knows this and He proceeded accordingly.
Rabbi Jack's book Ask Rabbi Jack is now available from Kodesh Press and on Amazon.com.