To be fair, the Cardassians weren't much of a threat when they were originally introduced in "The Wounded", which featured a single rougue Federation ship operating with impunity in their territory, along with a couple of other references in that episode that made it pretty clear they were at a technological disadvantage to the Federation. Only later when they were made the principal antagonists for DS9 did they get an upgrade to a major superpower, at which point a number of the introductory elements from "The Wounded" made less sense.The fact that it could be going on during parts of TNG and we don't hear about it at the time leaves an impression that the war wasn't all that significant and that the enemy wasn't an especially threatening one, despite evidence to the contrary.