We should have preferred to leave on the 8th , but outing to some reason we didn't. (划线部分错误)
We should have preferred to leave on the 8th , but owing to some reason we didn't.(改为 owing to,也是错误)
We should have preferred to leave on the 8th , but for some reason we didn't.
should have preferred to do:(过去)本应该优先做某事(实际未做)。
we didn't:正好符合前文的意思,指过去没有。为什么要用现在完成时呢?