re: Wasn't there a solar activity issue?
And lower altitude allows more payload to be deliver per launch. This has to be balance against the higher drag from lower altitude. Therefore with more quieter solar activity and thereby reducing the drag from Earth atmosphere one can have a lower the altitude.
Though the sharp drop in ISS in the beginning was during the time of high solar activity, therefore it wasn't optimal to have it lower- but it was also placed in the beginning at a higher orbit because this gave them more time to handle logistical delays of the next expected reboost.