Granted, it was also considered a "darker" content show than shiny happy TNG - which the average audience back then did not gravitate toward as a storytelling style. And the other main reason was the static/non-starship setting which turned a lot of TOS/TNG viewers off.
That said, I think it could be argued that DS9 is the best-written Trek series *as a whole* (and I say that with a deep-decades-long love of TOS), and is the least dated of all the shows in large part, as I said, due to the serialized storytelling and the fact that it can be an incredibly dark show, which is very current storytelling style.
Try showing a modern viewer an ep of TOS, TNG, DS9, VOY, and ENT. I think they would absolutely gravitate towards dark, story-arc-driven DS9. Unless you showed them what is generally considered the best ep of TOS - City on the Edge of Forever - probably the darkest episode TOS ever produced - a story of drug overdose, rehab and death!
Then again, i think Millennium is underrated for the same reasons - it was bright shiny X-Files darker, more serialized sibling. People were turned off of it quickly. But I think in comparison, Millennium (the first two seasons at least) is an incredible show that still feels fresh and could run today. It, like DS9, was probably just ahead of its time. IMO