Even though it was a relatively peaceful time (no Romulans, no Klingon threat, no Borg, no Dominion - but they did still fight wars with the Cardassians and the Tzenkethi), the Prime Directive would probably have prevented the liberation of Bajor.
That would be a truly draconian - and unjustified - application of the prime directive:
The bajorans were an interstellar species, so relations with them were permitted as per the prime directive;
The bajorans (whatever passed for a bajoran government) asked - or would have asked - help from the federation.
Not liberationg Bajor because it's 'an internal' cardassian matter is akin to recognising that Cardassia's conquest and occupation of Bajor is legitimate:
That the cardassians had the right to conquer Bajor and have the right to keep it occupied by violence - which contradicts basic federation values such as liberty.