DSN like it's parent show TOS, seems to have found a bit more life once it had finished it's original run. That is to say it has gained in popularity a bit, with quite a few saying it's the best of the Trek shows.
Serliased shows have been around for years long before the likes of DSN and B5. So the audiance can and will follow a serialised show. As for why it got lower ratings take your pick :
More competition from other genre shows
Dislike of the concept
Slow to get going (yet DSN S1-2 where better in quality that S1-2 of TNG)
We each have different tastes, perhaps part of the feeling as to why DSN is/was considered mistreated stems back to when it was first announced/produced. Star Trek on a space station how will that work, they won't go anywhere etc...
Yet Star Trek was never so much about the going better exploring the human side of the equation. Visiting the alien planet of the week just allowed the writers to get things past the censors which might not have gotten post them in a contempary setting (true not every episode).
In the end DSN was a success for Paramount, it made them money.