used to introduce a reason or explanation for the preceding statement (similar to 'because' or 'the reason is').
彼は来なかった。というのも急に熱が出たからだ。
He didn't come. That's because he suddenly had a fever.
試験はやめた。というのも、準備が間に合わなかったからだ。
I gave up the exam. The reason was that I couldn't prepare in time.
彼女が怒った。というのも冗談がきつすぎたからだ。
She got angry, because the joke was too harsh.
参加者が少なかった。というのも案内が遅れたからだ。
There were few participants, because the announcement was late.