在英语里,一句话中不能同时用why和because.
这句话应改为：the reason why he was late was that he got up late.
原因：这句话的主干是the reason is sth.
sth就是这句话中的由that引导的同谓语从句（少数人认为也可以理解是表语从句）,注意,无论是同谓语还是表语从句都得加“that”来引导!