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

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.