because; the reason is
Formal or written conjunction used to state a reason explicitly. Often followed by から or のだ. In casual speech, なぜなら is more common, and だって is used informally.
Because that is a fact.
私はその計画に反対です。なぜならば、費用がかかりすぎるからです。
I am against the plan, because it costs too much.
He passed the exam. That's because he studied every day.
This plan will fail; the reason is that funds are insufficient.