Abolish means to put an end to some previous activity (without legal, political or official legal action, documents or orders) Abrogate means to put an end to some previous activity (but with official legal action, documents or orders.)

abolish is a more gradual proceeding than abrogate

Basically, abolish is about customs, traditions and usual stuff, while abrogate is more concerned about law and legal things.

Some people don't want to abolish slavery. :P