Why would ABC/Steven Bochco release four seasons of NYPD Blue then stop?

There might be problems with the music rights. Shows that were made before they knew how big release to DVD was going to be, didn’t secure the music rights except for original broadcast and reruns. The cost to do so later is sometimes more then they are willing to pay. I haven’t read this about NYPD Blue but have read it about other shows that only released a few seasons on DVD and not the entire series.