Because it would have never worked as a film
The brilliance of “True Detective” is not only in it’s rich narrative and story structure. Not only in the great performances, absolutly stunning photography and production. It changed the face of television because the creators decided to tell clear a story from beginning to end, on television. An outlet not used to clear endings. Thus “True Detective” served the story and not the medium in which it was delivered.
This was a film, way too long to be a film. A TV show, way too short to go on further than a season. The character arcs, which is two detectives righting the wrongs on the biggest case of their lives, didn’t need to be dragged on any further. But it was too rich a story to fit into a regular two hour thriller film, there was a lot more to tell.
"True Detective" will live on as one of the greatest one season’s of any show in history, because they decided to leave it as one. I Don’t know if they will be able to top it next season but I cannot wait to find out. And to writers; learn from this show. Enrich your characters, clearly define their boundaries and settings. Give them something to do every frame and something going on, something to say. Especially if you are writing crime.