Why did it take an HBO doc...

I was thinking the same exact thing. Especially considering the dots seem so obvious. If the evidence is truly the letters, they had that evidence all along.

Part of it certainly is that he spoke to the filmmakers with a candidness that he never would have with the cops. In this case, he talked his way into trouble. Journalists don't have the constraints on them that cops do and people will say things/hand over things that they wouldn't to the police.

Could be for the same reason that Bill Cosby's only in trouble today because of Hannibal Buress. Some people only do their jobs when they know that other people are watching.

Because he was dumb enough to talk to them without a lawyer going over every minor question beforehand. He benefited from great legal representation until he was dumb enough to do this.

It all boils down to money. Just like OJ. If you have enough scratch you can buy freedom, while being guilty as hell. The next level is determining if the family was complicit.

---Listen, ..do you smell something?---

I also think it comes to "the whole world's watching" factor. This case being made a documentary exposes to a much larger audience who Bob Durst is and what he's capable of. Sure, I remember the Galveston thing and was outraged for a couple of days, and then completely forgot about it.
Regarding what's going on now: did the LAPD know about the evidence presented here (and then some)? I believe so; maybe they were coerced to look the other way/not make it a priority.
Docs like this can really serve to a)expose a corrupt system, and b)hold the justice system under a microscope (by a much larger audience) so it doesn't happen again...at least when this particular individual is concerned. Did the LAPD know about the evidence presented here (and then some)? I believe so, they were probably coerced to look the other way/not make it a priority. Another example of this would be the "Paradise Lost" documentaries. I believe 100% that these guys would probably still rotting in jail (one of them already executed) if that doc hadn't happened, if there hadn't been a record of what a complete travesty that whole trial was.

Each murder trial Bob Durst faces is made more difficult to win by the fact that a preceeding trial exist, regardless of his super expensive legal defense. Not impossible, of course, when unlimited re$ource$ and a complete lack of a moral compass (by both the defendant and the lawyers) join forces...but tougher.

Did the police have a copy of that letter written from Durst to Susan Berman? I thought Berman's son gave it to the documentary people and not the police. This letter is the smoking gun that they needed to make an arrest. Without this letter linking Durst to the cadaver letter, they really didn't have anything solid to arrest him with.

I do not think police were given the second letter that Sareb Kaufman (Susan's stepson) gave the documentary people. The police didn't have anything of Robert's handwriting to compare with the letter and probably didn't have enough evidence to get a warrant for it. Sareb seemed to go straight to Andrew Jarecki and co with the letter instead of the police.
This second letter was the smoking gun that was needed to truly connect him to the murder. Plus speaking to Robert with lawyers present would be useless. The documentary makers got to speak to Robert without his lawyer and gave him a sense of comfort that led to more opportunities (audio recording)

Dear Warden, You were right. Salvation lies within.