Why do Holy Days of Obligation differ by Diocese in the RC Church?

As I understand it, Ascension Day, for example, is a Holy Day of Obligation in Philadelphia but not in Erie, different ends of the same state. The Episcopal Church is even stranger with days of special devotion but no Holy Days of Obligation. Nonetheless, Anglo-Catholics in the Episcopal Church observe them and they are listed in the St. Augustine prayer book. Can't we all get together on this?