Fair dos - it wasn't for want of trying.
50 suspects identified by the public and followed up. Three arrests, but had to be released (had alibis, were out of the country etc). Loads and loads of publicity.
When the information dries up, the officers have to be switched to other investigations. You can't have them twiddling their thumbs, waiting for a lead.
The headlines are misleading. A case is never 'closed' actually. There will still be an officer with responsibility for the case. It just means that the rest of the team have dispersed and are working on other things.