Basically the title, I found it weird that we use two different names for each fields. What were the reasons for doing so, historically speaking?
Whilst we now call it electromagnetism, it was originally electricity and magnetism. They were seen as two separate phenomena. Hence the terms permeability and permittivity arose from two distinct fields of study that were only later unified.
It was Oersted in 1820 who first published a link between the two. He had discovered that a magnetic compass needle was deflected by a nearby electric current. This motivated research into electrodynamics, and, forty years later, in 1862, Maxwell published his equations that unified electricity and magnetism. It was one of the earliest unifications. Incidentally, Oersted is often said to gave discovered the link accidentally. This is simply myth. The truth is he was researching for a link between electricity and magnetism for around two years before he made the above discovery.
Whilst one could argue that the two terms - permeability and permittivity - should be unified into a single term, the terms remained as they were, as they had already been established for some time. In fact, recently standards organisations have moved to calling them the electric and magnetic constants.