because; the reason is
Used at the beginning of a sentence to explain a reason for a previous statement. Often followed by a clause giving the cause or background.
He can't come. The reason is that something urgent came up.
この店は人気がある。というのも、値段が安くて味が良いからだ。
This restaurant is popular. That's because the prices are low and the food is good.
From the quotative particle と, the verb 言う (to say), and the explanatory particle の, literally 'saying that it is that...'. The phrase grammaticalized into a conjunction meaning 'because'.