Why should you use a piece of lens paper only once?

The cleaning process is used to eliminate smudge and dirty grit. Because this dust can cause damage to the lens if reused, hence it is recommended that the wipe should be disposed of in order to prevent damage in later cleanings.

If you re-use it can have the possibility to smear oils from the fingers on the lens. Moreover, the dust from earlier used paper can damage the coating. Because the lens paper is like a paper sheet, once it is stained, you cannot use it again.