While I think DS9 was by far the best Trek Show ever made, I will be honest and say it was mainly two problems. 1. The characters weren't as strong or likable as on TNG. 2. The heavily serialized episodes. There's simply no way anyone at ALL could just "watch" a episode from season 4 and up without knowing what happened before.
Serialisation shouldn't be held as an issue, serilaisation isn't exactly uncommon when it comes to shows. If anything it it was the lacklustre quality of the early episodes. SUre on balance S1 of DSN was better than S1 of TNG but some in the audiance aren't willing to give shows a chance to find themselves which can take a season or two in some cases. Instead they might watch a few episodes before giving up on it. Which has now become a double edged sword as some are now unwilling to invest in a show esp. the more serialised shows in case it's cancelled after a season. So it ends up being cancelled after a season.