I don't get it either. I think DS9 had a stronger season 1 and 2 than TNG and Voyager. That's not to say there weren't duds, but nowhere near as many as people make out.
And DS9 didn't just "get better"...it specifically built on the foundations of season 1 and 2. They weren't treading-water seasons, they were seed-planting seasons. ...Mostly.
Mixed metaphors for da win!