Why is it asking Hong Kong to pass a law that so clearly challenges the freedoms the nation enjoyed and which the party said it would keep？

the freedoms （which）the nation enjoyed and which the party said it would keep

the freedoms被二个并列的定语从句修饰，第一个定语从句的关系代词在从句中作宾语，可以省略，但第二个定语从句的关系代词则不可省略。