You can start a sentence with “because” as long as you make sure it’s a complete sentence. Starting with “because” can create a sentence fragment, which may account for why some confidently admonish such beginnings. Let’s take a closer look.
“Because” is a dependent marker word. When you add it to an independent clause, it can become a dependent clause, meaning it does not express a complete thought on its own:
Because Franny took the El to the lake.
See how the thought is incomplete? What happened as a result of Franny taking the El to the lake? This is a sentence fragment.
Here’s how it might look as a complete thought:
Because Franny took the El to the lake, she couldn’t bring her giant beach umbrella.
So, if you’re going for a grammatically correct sentence, you’ll want to make sure a sentence that starts with “because” is complete. If you’re abandoning the grammatically correct, as some creative writers choose to do from time to time, make sure you’re doing so on the foundation of a strong stylistic choice.