First, welcome to the group and I am so sorry for the loss your community has endured. I really can't imagine how it feels to be local and to not know who did this. I personally don't think this was a random crime and firmly believe this was personal and Missy was targeted, but that is JMO. I wish the police would release more but part of the reason I feel they are confident this was not a random crime is because of how they have handled it in the media if that makes any sense. I don't think it's about showing the town that the PD has the man or woman responsible for this crime. They will do so in time when they feel they have enough to charge someone and get a conviction, imo. They should not feel rushed due to public pressure, everything I have seen, from an outsiders perspective, has been methodical. I think they are building a case against someone.
There are a lot of reasons why they might not arrest someone they consider a poi or suspect on a lesser charge just to get them off the street. But I have seen some other posters respond to this question with very good answers so I won't address that.
Also, in regards to LE not releasing too much and in regards to the recent presser regarding the suspects height and the Altima, I found some info from the the fbi on media relations during serial killer investigations and I wonder if some of the stuff in their recommendations for that could be applicable here as well even though this is not a serial killer investigation I still think it's worth typing out and providing the source, especially since the fbi has assisted in this case as well.
Basically, the FBI believes that local LE and/or whatever agency is handling the case, should design a proactive media plan, especially in high profile cases. The part I found most interesting is that they believe any press release or new info released on the case should be "closely coordinated with investigative strategies. This helps determine the best times to both educate and solicit information from the public concerning certain aspects of the investigation."
Further, the same source (FBI) talks about how press releases should have very specific objectives and there are different purposes they can be designed around some of which are to announce a development, educate the public, solicit info from the public, provide public safety information, to correct misinformation about the case, and to encourage someone who may know the killer to come forward. Also, they recommend that when verbal comments will be made to the press in conjunction with any written press release or update, they should be practiced with lead detectives first because they can end up negating the strategy of the written press release.
This all led me to re watch and read the latest presser, and with the info given and the fact there was no live stream, it seemed very controlled. It seemed like with the image of the Altima and the height range given, the point was to solicit info from the public and encourage someone who may know the killer to come forward. Because it is recommended that these releases are closely coordinated with investigative strategies it furthers my belief that they know who did this and are seeing how they react and/or if anyone who knows them comes forward, JMO
Here's the link, click on the part that says "media issues in serial murder investigations"
https://www.fbi.gov/stats-services/publications/serial-murder/serial-murder-1#nine