Why does it take spaced repetition to make the brain retain the memory? Is there another way that it could be shortened and easier to retain?

The principle of spaced repetition is that it addresses the ‘forgetting curve’ which is the loss of information that you have just studied.

This was first described by the German psychologist, Hermann Ebbinghaus. The sharpest decline occurs in the first twenty minutes and if you forget much that you have learned in the first hour after committing it to memory you need to follow a schedule of spaced repetition which was first published in a paper by Paul Pimsleur in 1967.

IT IS NOT THE SAME AS REPEATING EVERYTHING OVER AND OVER and so I prefer to call it Spaced or Interval Reviewing, which is reviewing what you have learned at spaced intervals - just at the point when you might forget it. An example of spaced reviewing would be 10 minutes, 1 hour, 1 day, 5-7 days, 1 month, 3 months, 6 months, and 1 year.

So, if you are going to forget much of what you have learned within an hour, I cannot see any way of shortening that period.