The thing is, this is not exactly for lack of trying on the killer's part. Since he was responsible for at least 50 sexual assaults but "only" 12 murders, he was seen by plenty of eyewitnesses. We've got enough police sketches to populate half of Skyrim with custom characters based on them.
"Case closed -- the two oldest Hanson brothers and Bill Pullman did it. Cracking job, everyone!"
Also, despite clear tactical prowess and active attempts to operate stealthily, he was often the exact opposite of careful with his crimes. When he broke into his victims' houses to tie them up and either shoot or bludgeon them to death, he would sometimes alert them by repeatedly muttering "I'm gonna kill them all," which is the sort of mysterious noise from downstairs that generally alerts even the thickest homeowner. He would attempt to flee crime scenes on a fucking bike, and nearly get caught by angry neighbors. At one point, he may even have taken a huge-ass dog with him to murder people. (Just to tag along; not to do the actual murdering.)
Oh, and he also actively kept in touch with the media, the police, and his targets. Here's a creepy-ass phone call he made to an intended victim: