Here's why:
* Subordinating conjunctions connect a dependent clause (which cannot stand alone) to an independent clause (which can stand alone).
* "Because" introduces a dependent clause that explains the reason for the statement in the independent clause.
Example:
* Independent clause: I stayed home.
* Dependent clause: Because I was sick.
The entire sentence is: I stayed home because I was sick.
The dependent clause "because I was sick" explains why the speaker stayed home.