David Simon shows don't make bank for HBO but their good for critical acclaim.
Remember The Wire technically got canceled after season 3
If you notice the last few shows David Simon has been involved in except Treme have been 6 episodes no way they'd ever realistically fund multiple seasons.
Sadly the audience for non-Law and Order style procedurals is small.
These David Simon police shows tend to be denser and slower, with no good guy getting the criminal happy endings, they have an audience but it's relatively small.