Here's why:
* Prefixes are added to the beginning of a word or root stem.
* Suffixes are added to the end of a word or root stem.
Therefore, a syllable or word element that can only occur in front of a root stem at the end stem must be a prefix.
Example:
* Un is a prefix that can be added to the beginning of the word "happy" to create "unhappy".
Let me know if you have any other questions!