It had a lot to do with the internal politics of Cardassia, mainly the rivalry between the civilian branch of the government and the military leadership. This was explicitly adressed in the Season 2 episode "Cardassians". Perhaps the civilian leaders were always opposed to the occupation, while the military was a strong proponent and the driving force behind it.
Something that wasn't actually adressed (not even in "Cardassians") was how the civilian leaders were finally able to force a withdrawal from Bajor against the military's objections.
Something that wasn't actually adressed (not even in "Cardassians") was how the civilian leaders were finally able to force a withdrawal from Bajor against the military's objections.