Explain briefly why you cannot expect to find a previous version of every file with which you work.

Our file can be easily overwritten by a single click of save button. Many times we may only work on one single file and save it from time to time. Every moment we save our work, the previous copy of work will be replaced with the current version of work. This is why we can't always find a previous version of every file we work earlier. If we expect to retrieve previous version of work, we should save the latest work into a new file instead of overwriting it.

Most people will save files on top of themselves, overwriting their previous versions.