Probably how you define exploration, I suppose. I don't consider DS9 to be an exploring show apart from a few episodes where they potter through the wormhole. Voyager is just going home, but I guess you could argue that counts.
Planet or creature of the week would become dull, you have to look at those other missions in the context of the hero ship being part of a whole....colonies, as depicted, are very much exploration, the ships support those. Ambassadors, depending on their brief, are also very much part of exploration, so the ships support those...once you break it down, there are very few non exploratory episodes. Basically the military ones, unless it’s as a result of First Contact or similar circumstances (as DS9 is serialised, it’s entire war is looking at the results of contact with the Dominion.) Basically....Arena in TOS is exploration because of following up on the colonies, whereas as almost every Romulan or Klingon episode in every series is not...because those are not new races to the Federation. ENT and now DSC could redefine that in the way DS9 works, but I don’t think the Klingon Wars come out of botched First Contact, and certainly would be hard to redefine as that, because the Dominion works as exploration story telling precisely because it’s linear...we meet them, both sides fuck up contact, it leads to war (perhaps inevitably because of the Dominions approach.)
The Borg stop being about exploration after their first appearance, but if generous can be handwave because much of their TNG appearances involve supporting colonies or new groups (Hugh). After that though, unless the story is explicitly tied, they are just a recurring threat (the Hansens we’re exploring)
None of the TNG films were exploration, but then, Picard opines as much. Sure, they discover new things, but not entirely by looking for them. But then...only two of the TOS films had much ‘exploration’ and both of those had other reasons for bringing the unknown to the heroes, rather than them going to it. TMP and Final Frontier Basically.
It’s a messy thing, when you get into it.