While my various groups and I managed to play in every Marches' subsector, I set a majority of what I did in the "Four Corners" region; Swords - Lunion - Glisten - District 268.
I'd originally picked that area for a couple reasons. Grote was one region; a delightfully odd little system with the one of the few Class A ports in the region. The Imperial border was another reason as were the Swordies and all the "Indies" in the District. After I'd picked the region and begun creating materials for my use, there was a resulting "inertia" of a sort which played a big role in keeping me there.
Eventually I had so much material, I had so many world descriptions, NPCs, companies, rumors, governments, you name it, that I could easily drop a new adventure, set of sessions, or campaign in the previously detailed region. What was helpful at first, true to my nature, quickly became a crutch.
I became so used to using the region that I deliberately set my later Active Duty IISS campaign in Trin's Veil as a way to shake up my thinking.