Characters spoke in heavy slang which was rarely explained. Plus, even with the white characters, there wasn't a lot for the casual watcher to latch on to. Take Season 2: it still got the highest ratings of any season, but it was all about dock workers and the death of American industry. Not something Hollywood can really relate to.

Then there's the casting: especially on the "good guys" side, they weren't a lot of "pretty" people that they could market to People Magazine. Cops and judges tended to look like actual cops and judges.

Plus, at the end of the day the show was about the fate of the city and urban decay. Sure, there were over-arching issues each season, but nothing ever got neatly resolved before moving on to the next season.