I think it's because like the stuff you learn in modern (politics and other stuff) is closely interrelated with what you learn in law. Lol not completely sure, took a stab.
well modern is very demanding in terms of memorising and essay writing, much more than legal i think. plus a lot of the stuff learnt in modern somewhat affects law today so u can understand it better
whereas legal studies is NOT the study of law itself, more of legal processes and important case studies. Just to clarify things out