Because the murderer will see the news too?
IMO, they want to avoid alerting their suspect of what evidence they have, where the investigation is at. They will keep the person under constant surveillance, so they aren't risking another crime. Usually, they are still building a case to arrest the person, but they don't yet have enough evidence. They hope the suspect, unawares, may say, or call, or do, or lead them somewhere, or they're still following up other leads, such as cellphone data or a computer...
If they had to prematurely make an announcement and all they have is a 'person of interest', their ability to gather more evidence might be stalled.
An example of this was Bruce McArthur in Toronto. Police were watching him around the clock (but they were still analysing evidence, hoping to find the bodies), but they arrested him when a potential victim entered his home. As it turned out, they did later find the bodies, and in the end he confessed.
But they don't want to risk not having enough to nail the guy, and having to let him go. People will definitely complain about that.
JMO