Here's why:
* Subject: It lacks a subject, the person or thing performing the action. We need to know *who* is doing the action.
* Verb: It has a conjunction ("because") but no main verb. We need to know what is happening.
To make it a sentence, you could add a subject and verb:
* "I did it because you said so."
* "He left because you said so."
* "The dog barked because you said so."
Let me know if you'd like to explore other ways to make a sentence!