The reason why he has not come is that his mother was sick.主句从句时态一致，这里不应用现在完成时吗？我觉得两种都可以，这个是表语从句，不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。 另外你填 The reason why he has not come is thathis mother has been sick& ill. 也对