이었으니까

because it was, since it was

→ 이다
이 to be + 었 Past tense + 으니까 Because; since
auxiliary · informal, past tense Uncommon #14,707
because it was, since it was Used after a noun to give a reason.
KO: 어제는 휴일이었으니까 집에서 쉬었어요.
EN: Because yesterday was a holiday, I rested at home.
Conjugation of 이다

어쨌든 해적들 이었으니까.

Well, they were pirates, after all.

그게 계획 이었으니까

Because that was the plan.

— 블레이드 러너 2049 (2017)

네 아들 이었으니까

Because he was your son.

— 내 어머니의 모든 것 (1999)

물론 뜨거운 전 이었으니까.

Of course, it was a hot one.

금으로 된 게 아닐 거 예요 목수의 잔 이었으니까

It probably wasn't made of gold, since it was a carpenter's cup.

— 인디아나 존스: 최후의 성전 (1989)