There are lots and lots of scenarios that might make them not want to do a forensic exam of the car. The easiest way to explain the idea is by an example. Suppose the POI's mother knew everything that happened. I mean, in every detail. Suppose she tips off someone in dribs and drabs. Eventually a picture emerges that JEW was taken outside of town, that he bolted from the car when it stopped, that the POI got out and shot him in the back, and that JEW died less than one hour after being taken. That he was buried in this same area and that little actually happened in the car. Now, you know this mother passed this and some "souvenirs" she took from the POI on to someone else. Then she died. The key detail she never divulged is where are the remains. But some of the details given corroborated what LE knows independently, so they know this narrative is probably true. But you need the witness to provide more to file charges. Do you endanger or alienate the witness by searching the POIs car, which might tip off the POI that he has been ratted out? You can think of dozens of scenarios where they would not want to request warrants, each one speculation, but each showing a good reason for the current state of affairs.