that is why后面跟的是结果,译为：“那是（为什么）.的原因”
例如：
Tom overslept this morning.That is why he was late for work.
汤姆今天早晨睡过头了.那是他上班迟到的原因.
that is because后面跟的是原因,译为：“那是因为.”
例如：
Tom was late for work this morning.That is because he overslept.
汤姆今天早晨上班迟到了.那是因为他睡过头了.