Here's why:
* Subject: A sentence needs a subject, which is the person or thing performing the action. "Stop" doesn't tell us who or what is stopping.
* Verb: While "stop" is a verb, it needs to be part of a complete verb phrase to make a sentence.
To make "stop" into a sentence, you need to add more information:
* "Stop!" (This is an imperative sentence, a command, where the subject "you" is implied.)
* "The car stopped." (This adds a subject, "car", and a complete verb phrase, "stopped.")