Perhaps they know who the phones belong to, but what makes them think the owner is one of the San Bernadino killers?
That's where law enforcement is having a hard time.
* Government can use a warrant to demand the item be surrendered, and preserve it as evidence.
* Government can demand passwords from third parties like phone companies under both subpoenas and warrants.
* BUT individuals have a constitution protection against compelled self-incrimination.
The government is supposed to produce evidence and link the person to the crime without a forced confession. It is a GOOD THING, it helps prevent things like being tortured to confession and fishing expeditions looking for crimes. Prosecutors and police can demand an individual produce papers and documents that link them to a case, but (assuming their legal defense is doing their job) by doing so they trigger the protections of the fourth and fifth amendments by compelling the evidence.
This was recently re-affirmed by the supreme court in US v. Hubbell.
If the government demands that the person gives up documents, papers, or passwords to the device it is compelled self-incrimination. If the government demands a person incriminate himself to collect evidence, it becomes poisoned and the government cannot use it or information from it to help with prosecution.
Police and prosecutors absolutely can demand the people turn over passwords .... but by doing so they also trigger immunity, they cannot use that fact or anything learned from the devices as evidence against them. They'll bitch and moan and complain about not having the passwords, they'll petition congress about how unfair it is to law enforcement that police need to actually investigate crimes and can't use self-incrimination tactics, but the lawyers know full well all it takes is a single slip of paper to legally demand the passwords. Grant them immunity under the protections of the 5th and they are compelled to turn the passwords over, but the person also walks away from criminal liability.
Simply (perhaps dangerously oversimplified) in most of these cases it is that the police are lazy. There are many other known details, much other evidence, but investigators are going for the easy pickings of the data on phones and other personal documents typically protected by law. They could do actual leg-work, actual investigation, actual crime scene evaluation, and many investigators do. The ones wanting to break down the constitutional protections are the lazy investigators who won't be bothered to use the other available investigation tools.