Li Ming is late,as is often the case.为什么不能用which

这题用which引导非限制性定语从句也可以,但是as is often the case常被看做一个句型,还有as was expected等。如果不是固定句型,as常是放在句首的,which常是放在句中的。下面是一道上海高考题,选which引导非限制性定语从句：His movie won several awards at the film festival,(which) was beyond his wildest dream。