Here's why:
* It lacks a subject: We don't know *who* or *what* the sentence is about.
* It lacks a main verb: It only has the conjunction "because" and a dependent clause.
To make it a complete sentence, you need to add a main clause. For example:
* The frog was heavy yesterday morning because he ate too many flies.
* I was surprised because the frog was heavy yesterday morning.
* Because the frog was heavy yesterday morning, he had trouble jumping.