I WAS gonna chime in on this but then you mentioned my nom so. . . you're ABSOLUTELY correct, clazor!!! 
I think CR states the main reason; it IS a movie of Laura along with a character study of those around her and the mystery DOES come secondary. It is easy to guess the killer, though I'm sure in it's day Price's "red herring" kept many a movie-goer a-wondering.
Along with that there is some truly excellent dialogue, and for me that really makes a difference.
I do have a very long-time love for the movie so that may cause some bias for me though even on my recent watch still found myself loving this movie. so. . .

I think CR states the main reason; it IS a movie of Laura along with a character study of those around her and the mystery DOES come secondary. It is easy to guess the killer, though I'm sure in it's day Price's "red herring" kept many a movie-goer a-wondering.
Along with that there is some truly excellent dialogue, and for me that really makes a difference.
I do have a very long-time love for the movie so that may cause some bias for me though even on my recent watch still found myself loving this movie. so. . .
__________________
What I actually said to win MovieGal's heart:
- I might not be a real King of Kinkiness, but I make good pancakes
What I actually said to win MovieGal's heart:
- I might not be a real King of Kinkiness, but I make good pancakes
~Mr Minio