以下也是原因状语从句; As / Since he is ill,he can't come.
同样,它也可以变成强调句：
It is as / since he is ill that he can't come.
所以,依据是：在英语句子中,你想强调的任何部分都有可能成为强调句的一部分.The question is whether you will do it or not,breaking out of the so called grammatical rules.
我在一本习题上看到说在强调句式中,若强调原因状语从句,只能用because引导,即it is because.that.