the reason for this表示“这个产生的原因”.只是一个名词短语.
that's why表示“这就是为什么会.的原因”.why后面应为完整的句子.
放到语境中更容易理
He was late for class this morning. The reason for this is that he got up too late.
他今天早上上课迟到了.迟到的原因就是他起床晚了.
= He got up too late. That's why he was late for class this morning.
他起床晚了.那就是他今早上课迟到的原因.