如果没有was,整个句子的谓语就没有了
it是形式主语（we could not go out是主语从句即真正的主语）,还有这句中间不应该有逗号
because the heavy rain在这里是错的,正确的应该为“because of the heavy rain”这个介词短语做表语,was是谓语
正确的句子为It was because of the heavy rain that we could not go out.