你的問題包含了兩個(gè)問題.
1.Why 能不能引導(dǎo)主語從句.答案是肯定的.
例句：
Why he did this was a mystery to him until he tried working with affirmations and blurts.
選自Julia Cameron 《The Artist's Way》2002.
2.有沒有“Why+從句+is because+從句”這樣的結(jié)構(gòu).答案是幾乎沒有,至少在地道的英文里沒有.這已經(jīng)幾乎屬于洋涇浜英文了.拿你舉的例子來說,
Why he was late yesterday was because the traffic was heavy.（注意主句和從句的時(shí)態(tài)要一致）
表達(dá)不好的人才會(huì)這么說.其實(shí)簡單地說
He was late yesterday because the traffic was heavy.
更清晰也更符合英語習(xí)慣.