I'm not going to claim to be able to do all the math involved, but I am aware of some issues some others apparently aren't aware of:
- The higher orbit the more propellant it takes to reach that orbit and return to Earth from that orbit. The practical result often translates into an inability to supplement already full tanks, thus the weight of the cargo (and or the module) must be reduced in order to reach that higher orbit. Since US shuttle participation was considered necessary the chosen altitude already precluded station visits by the heavier Columbia (ironically equipped with more/heavier re-entry thermal protection)
- The station's orbit is at a significant angle to Earth's equator. To a great extent this is what prompts the brief "launch window"periods for spacecraft planning a visit. The visiting spacecraft needs to be launched as the Earth's rotation brings the launch site under the station's orbital plane (a few minutes twice a day). There are sometimes other constraints to launch opportunities like lighting (at launch and/or abort sites), weather and clearing shipping from areas where boosters will fall. If a spacecraft is in the wrong orbital plane it would take an enormous amount of propellant to change its orbital plane.
- While there were some periods during early construction when the station was unattended for months at a time, I'm not sure it can still operate without crew on board to occasionally adjust and/or repair some of the equipment. I've heard some of the oxygen generation equipment is particularly delicate and not completely understood by currently available personnel.