I wouldn't necessarily call Nancy, Greta, or Geraldo investigative reporters. They have their own TV shows that focus on issues (usually crime) and unfortunately, they don't seem to have much interest in Hailey's case. To me, Nancy, Greta, and Geraldo are the national media, not investigative reporters so the real question is, "With the local media actively covering this case, why hasn't the national media given it more attention?"
I heard once, that the farther a case is from a major media market, the harder it is for the national media to cover the case; Colorado City is 4 and a half hours from Dallas. It's harder to get into these rural areas, and there aren't as many hotels for reporters to stay in, or the only hotels available are 2-star chain hotels. I also think that's a reason why we didn't see much national coverage of Taylor and Skyla, the two girls in Oklahoma who were shot and killed.
I could be mistaken but I think the only national coverage this case has gotten was NG in the beginning. There's been sensational developments since NG dropped the case, but she hasn't featured it again. Why? There doesn't seem to be one big case going on right now that would take up all of her episodes so I don't know why we haven't seen a new NG episode about the CP, etc.