Here's why:
* "Because" is a subordinating conjunction. It introduces a dependent clause, which cannot stand alone as a complete sentence.
* Dependent clauses need to be attached to an independent clause. The independent clause provides the main idea of the sentence.
Example:
* Correct: Because it was raining, we decided to stay inside.
* Incorrect: Because, it was raining, we decided to stay inside.
The comma after "because" would incorrectly separate the dependent clause from the independent clause.