She got the promotion.
She is good at her job.
If the first sentence is a result of the second sentence, there are three ways to write it:
She got the promotion because she is good at her job.
She got the promotion as she is good at her job.
She got the promotion, since she is good at her job.
In an absolute grammatical sense, all three sentences are correct. But, as with all self-proclaimed language mavericks, we are uncomfortable with “as” and “since” used as causative conjunctions.
Of the three conjunctions used, “because” is the only word that has a single purpose. Its purpose is uniting the cause and the result – it is a causative conjunction alone.
“As” could serve two purposes. One, as a causative conjunction, as seen in the above example. The other, as a time-indicator. For example, “I walked into the station as the train was leaving.” It is also used in lieu of "like". E.g. Do as I do, not as I say.
“Since” is much like “as”. It can be used as a causative conjunction or a time indicator. E.g., “I have loved reading books since I was four years old.”
We reiterate – it is not grammatically wrong to use “as” and “since” as causative conjunctions. However, when you have a word that is dedicated for the purpose, why use another word that could potentially be understood differently?
There is, however, one conditional guideline for the use of "because" in formal communication - like all conjunctions, it is preferred to be not used at the start of a sentence. "Because the sky is blue, it makes me cry" is alright for the Beatles. For the rest of us attempting formal communication, especially in the sciences, it is "I cry because the sky is blue", although I suspect such an event would involve the use of recreational drugs.