My guess, no direct evidence against him.
I hope if they do know who it is it was an ex-con and they were able to take him off the street on parole violation like Ricci but there was no one like Mohl with Ricci to make it necessary for them to make it public. Thus they could keep him locked away until they either find Elizabeth or her body is found and they can continue gathering evidence.
It would be comforting to know that this person is safely put away. I do hope that someone in Ricci's inner circle of friends will give LE the information they need to get this guy. The Smarts have been through so much. I pray that they can find closure. My sympathies also go out to Angela and Richard's family. This has been an ordeal for them as well.