I think they shot themselves in the foot with the whole reset for each season idea. That they got such incredible chemistry between the two stars in the first series was probably way more than they could have hoped for and that chemistry is helping put season one on such a high pedestal. They are trying to throw the same kind of dialogue at the season 2 cast and it's not having the same effect, Vince Vaughan is just not in the same league as Matthew McConnaughey and his high brow dialogue is just not working.
Another thing is the story itself. In season one the crime was a lot more focused and disturbing which really made you want to see him caught, it was all that mattered. In season two though, the crime and corruption is so widespread that it's very difficult to know what they are trying to achieve or even if they are really trying at all.
I'm still enjoying it though, even if it doesn't have the magic of season one. Colin Farrell has really stepped up in this and is the obvious stand out but good as he is, his character is just not as likeable as Rust from season one.
Personally I think "Murder in the First" is absolutely killing "True Detective" at the moment, it's central characters have great chemistry and they also have a very good previous season to build on, so there is more depth to the story and they don't need to spend time building character back story. I certainly look forward to seeing it more than I look forward to TD every week.