Hauling unprocessed ore into orbit to be refined seems extremely inefficient. The Cardassians occupied Bajor for 50 years, so they had plenty of time to build refineries on the surface.
I believe that the Bajoran Uridium ore didn't have to come from the planet Bajor. Maybe it came from another astronomical body in the Bajoran system, a body with a small escape body so that the total escape energy and interplanetary transport energy would have been smaller than the energy to lift the ore up from Bajor.
Possibly the Cardassians only abandoned the Bajoran system after they had mined and exhausted all the valuable Uridium and other ores in the various bodies of the Bajoran System.
There is no canonical proof of this theory.
Evidence in favor of it is:
1) that it takes much less energy to mine an small astronomical body and move the ore to a station in orbit around a planet than it takes to mine the ores on the planet and move them up to an orbiting space station.
2) In "The Nagus Jake is teaching Nog to read and Nog says:
The lar, largest planet is Bajor. It has three moons?
3) in "Progress" there is the following log:
Station log. Stardate 46844.3. With the help of the Federation, Bajor is about to commence its first large-scale energy transfer, the tapping of the molten core of its fifth moon, Jeraddo.
So Bajor's moons have numbers up to five (or higher) and yet a probably nearly contemporary source says that Bajor has three moons.
Thus it seems that the number of Bajor's moons changes. If Bajor had at least two small asteroid sized moons full of Uridium ore the Cardassians may have totally demolished them to get the ores, leaving Bajor with just three remaining moons.