the reason is that ...; because
Used to introduce an explanation or reason, often at the beginning of a sentence or clause. Functions as a conjunction-like expression.
The reason is that it rained yesterday.
He was late. The reason is that the train was delayed.
A phrase composed of なぜ (why) + か (question marker) + と言うと (when one says). Literally 'if one says why, it is that...'.