That's why...引导的不是定语从句,因为why之前没有定语从句结构中的先行词；why出现在is之后,故它是表语从句.定语从句的结构应该是类似于下面的句子：
That is the reason why...
The reason why ... is that .
例如：
That is the reason why (=for which) he arrived late.
The reason why(for which) he was late was that his flight was delayed.