TOS thrived on the strength of the chemistry between the main characters. The premises of the episodes were incredibly base scifi 'what-ifs'. The only thing that sets it apart form other science fiction shows is that it approached it from the standpoint of the logic versus humanism.
It's true DS9 was originally judged because the premise was different, but that's no longer the case. The fact that it doesn't have a ship is not a concern to new fans.
But the plain simple fact is, there are 20 million TV fans who want an idealistic adventure show and 3 million who want a dark show that criticizes human nature. That's the only reason for difference in the ratings.
TNG and DS9 are both spectacular shows.