the reason--he didnt come was--he was ill.填什么,为什么

the reason__he didnt come was__he was ill.填什么,为什么
WHY和WAHT.
第一个空,先行词是REASON的时候 引导词不是WHY ,FOR WHICH,THAT,WHICH这几个吗,为什么一定要用WHY,不可以用WHICH》》