because; the reason is
Used at the beginning of a sentence to introduce a reason or explanation, often followed by から or のだ.
Because he is still a child.
私はその計画に反対です。なぜなら、費用が高すぎるからです。
I'm against the plan, because the cost is too high.
He took the day off. That's because he had a fever.
This plan is unworkable, because there isn't enough budget.
なぜなら is a formal, logical connector meaning 'because' used mainly in writing, while なにせ is colloquial and often carries a nuance of 'after all, as you'd expect'.