Orbital ore processing is such an odd concept in a lot of ways. While not clearly stated, it is implied that the uridium ore that Terok Nor (later known as Deep Space Nine) processed came up from Bajor's notorious mines. But, even if antigravs are amazingly cost-effective, why ship bulk ore to the tune of several kilotons per day up to orbit?
Seriously, stop and imagine having humanity's primary oil refinery in orbit, today. All those oil tankers criss-crossing the planet would instead need to be launched to orbit, with the products dropped (gently!) back down for use. You think you pay a lot for gasoline now?