Silicon is critical for production of energy cells and some tech items, ore is vital for making weapons. There's just handful of sectors that have both types of asteroid in abundance, thus the biggest complexes are built there.
In AP, by making nice with the Yaki (as opposed to blasting the hell out of them as I did in TC) I have access to (and built some enormous weapon complexes in) Weaver's Tempest and Savage Spur. Aldrin qualifies too but the sheer size of that sector makes bringing together the necessary asteroids hopelessly impractical. At least in vanilla anyway!
