Well to begin, police business is police business. Policy dictates that most things that go on within a police department is to be held with the greatest confidentiality, for the privacy of crime victims, criminals, and the police themselves.
Now you may be sitting here, thinking at me, "But if the public was informed what is going on, cases would be solved quicker!!!1!" That is the civilian way of thinking. Evidence, theories, and suspects must be kept entirely to the police. The media has no right to know and society is better off without them knowing.
Just take a second to think a moment here and lets pretend that they do talk to the public about everything. The police, after conducting a crime scene investigation goes up to the reporter on scene and goes "Well from what we can conclude, the unknown suspect of the crime wears size 11 shoes, has a dog and a cat, and seems to have left tiny pieces of tree bark that we suspect came from his gloves." You know what would happen in this scenario?
a) The tree-cutter would quit his job, get rid of his pets, and wear oversized shoes to mask what the police has identified. Then after a good 10 hours of cleaning to get rid of all the pet hairs, he goes and throws his gloves into his BBQ and grills up some food as an alibi. Then once he is done with dinner, he goes down to the Goodwill with all of his shoes and donates them "from the goodness of his heart".
b) The public knows all the details of the case, so they can rehearse and confess falsely to a crime for whatever purpose (paid off by real killer, homeless & in need of shelter and food, just wanting to take the glory of a serial killing or whatever).
Police withhold information from the public so that when these murderers do commit crimes and are brought into the station, they can grill them until they slip up with a detail only the murderer would know and thus get busted.
Oh and by the way, awful source. Really horrible. Never bring up sites from .co.uk sites. Those are tabloids, not news. And TheGuardian is especially horrible.