Repeating a beatmap causes you to memorize the patterns in that beatmap. When you memorize the patterns, you are no longer learning how to read patterns better when you play that map. You can still get some improvement from playing it, but the improvement will not be as fast as if you didn't replay the same song a lot.
Personally, One of my areas of strength is sightreading beatmaps. When I play a beatmap, chances are I won't play the same one again for a few weeks at least, sometimes I don't replay it for a few months. There are maps I play more often than that, but even then I usually play my favorite maps less than once a week on average.