It's just a horticultural practice. Substrate quality will generally degrade with time, mineral loss/gain, breakdown of the media, and so on. If you have a plant that dies in a medium for an unknown reason, you don't want to use it again because you may introduce the pathogen that killed the plant to any other number of plants.
If you must re use, then I'd suggest that you start out with a good quality medium, rinse it with clear water (let it go through the rain for a while or so,) and to sterilize it. Of course, this isn't the right steps for non-cps. Media can last a good long while, but is best when you keep as much as you can with the original plant.
You shouldn't ever run in to this problem anyway because if you are repotting because there isn't enough space for the roots of a plant to grow, you should take as much of the media as possible as to not disturb the roots (barring aspects that'd make you want to dump the media) OR you're repotting because the media sucks, in which case you don't want to plop another plant in to it!