Basically, the documentary would have sucked if they included some of the evidence that they excluded... like the *67'ing her phone and him requesting her several times to come out and take pictures. Why exclude those facts? Because it would have been another murder documentary about a murderer that was obviously guilty by the end.
The documentary makers learned a lot of new information during the trial. I believe they left these out because they were already invested in the project and wanted to leave things ambiguously for dramatic effect. The real story is Dassey here. They should have dug more on his side, but it was kind of a sidebar to the whole story.