But if you watch "Paralellels", look at Data's explanation for Worf's condition. From there it can be theorized that "Dark Mirror" takes place in an alternate Mirror Universe that split a some point after "Mirror, Mirror", where in one universe the Terran Empire continued and in another it fell.
If there is more than one universe with the Federation ("Parallels"), there can be more than one Mirror Universe (e.g. the new alternate MU from the IDW comics).
Lacking the scientific background, I consider all the universes featuring Earth stemming from one source, with timelines multiplying like Tribbles (MyrU: "Places of Exile"). Maybe they remain physically in the neighbourhood, thus explaining while our heroes usually cross into similar universes often and other spaces (fluidic space, transdimensional space) rarely.