I also think it was a matter of the PD. Cardassians took Bajor, so it was their internal matter. I'm sure Bajorans weren't the first ones for Cardassians, and that Cardassians were not the only power with aggressive politic toward others. Klingons, Romulans - they probably had subjugated some worlds too. The Federation couldn't go from one system to another to liberate everyone.
In a war, in theory and in practice,
all targets whether civilian or military of an enemy are subject to attack. Cardassia had annexed Bajor, and was exploiting and brutalising its people. So, as Bajor was then Cardassian territory, and the Federation was at war with Cardassia, liberating Bajor may have been feasible.
A comparison with the Klingons or Romulans makes little sense. The Federation couldn't use war as a pretext/justification for liberation. By the time of the Cardassian Wars, the Klingon/Federation Alliance had probably just been signed, or at the least the first Khitomer Accords of Kirk's time was still in effect. So in either way, they were allies, and hence not subject to Federation attack. And obviously the Neutral Zone/Algeron Treaty had the Romulans in check (as well as their own isolation).
Does anyone consider that it might be really, really difficult to take over a whole planet? I mean, conquering a country is hard enough, but a planet? Oh boy! Not something you do unless you really have to.
Anyway, it would be so out of character for the Federation and so un-Star Trekky that it really doesn't bear thinking about.
When there are billions of Bajorans who would lend a hand, that makes it easier, doesn't it?

A Starfleet invasion force, plus billions of Bajorans, would greatly outnumber the resident Cardassian forces.