It's usually because of the writing.
In this case, it's the performances that drag this down, for me.
The Good:
Six episodes aren't a huge investment.
There are great performances from secondary characters.
Donald Sutherland!!!
Hugh Grant is very good, in this
The Bad:
Nicole Kidman. From the theme song, to her strange acting choices (body language, vocal cadence, etc), I wish someone else was cast in the role.
The Son. I'll blame the director here, because this young actor seems to perform in ways that were woefully unconvincing.
The Detectives. Again, from the start, the choices made for the actual performances are strange, and often nonsensical.
The Finale. It's a letdown, for sure, Not much more to be said.
But perhaps the worst part of this limited series, is that for 5.5 episodes, it kept me mildly interested in seeing the story reach its conclusion. And when it finally did, I legitimately felt an annoyance that I had wasted my time.
Another commenter used the Law and Order analogy, and that's pretty accurate. Outside of the brief nudity and language (and obviously the A-List cast), it really comes off like your average L and O episode-only with that show, it's just a 60 minute time investment.
Perhaps this might have made a better 2 hour movie.