因为这是个强调的句式.
it is/was.that.
我把句子给你还原你就明白了.请看下面：
It was 3 years ago that I last met her in the countryside.就是在三年前我在乡下最后一次遇见了她.
How many years ago was it that I last met her in the coutryside?多少年前我在乡下最后一次遇见她?
本句how many ..引导的宾语从句应该是陈述句语序.所以中间就填 it was that
现在明白了吗?