Should we allow reuse a image in a publication as long as it is properly cited and noted? Sometimes it is easier for the reader to understand a image if it is reused, even in the same publication. Just like we sometimes need to go back to a slide when doing a presentation.
What is the original reason for forbidding this? My guess is to save some pages. But now it is digital and manuscripts are already long and large.
Sometimes it is easier for the reader to understand a image if it is reused

Not really. You can go back and forth when reading a text. I see no advantage reproducing an image within the same document.

What is the original reason for forbidding this?

It is not strictly forbidden. I would call it common sense. The fact that you do not see images reproduced in the same document is because most people are convinced it is totally not necessary.