It Was The Gravitic Isolines This Whole Time...

Heh... there's a canon explanation for that, actually. Fights take place at and around jump points, and jump points lie at the intersections of isolines of graviton flux. Debris collects to and flows along these like plastic in the ocean.

So every battle takes place in the Great Pacific Garbage Patch, essentially. :) It's the same reason our asteroid fields are so absurdly over-dense. (A WC asteroid field is roughly as dense as Saturn's rings.)