since then是副词短语,不能引导一个定语从句.
“The book was written in 1946”和“the education system has saw great changes”都是完整的句子,中间肯定是用连词连接的,副词没有这个功能.
定语从句有介词,一般我们都是用介词+which的,但是这个句子比较特殊,用一般的思路不好理解.我觉得可以这样想：
which在定语从句中是要充当成分的,像是主语或宾语.但是看后面的句子“the education system has saw great changes”,主谓宾是完整的.缺状语,那就用when,不用which了.