Why on Earth did they name this "Cthulhu"? That's like Peter Jackson doing the Lord of the Rings trilogy, but naming it "The Hobbit." Right author, completely wrong story.
The word is a lot more identifiable to people. I've only read a little bit of H.P. Lovecraft, but I definitely know the name Cthulhu. Had they named it after one of his books, I wouldn't know it from any other book unless they had in big, bold letters "Based off the book by H.P. Lovecraft." Cthulhu, now I definitely recognize THAT.
That's what I assume.
But more to the point, the vast majority for whom the term "Cthulhu" is actually a draw--that is, more than simple recognition, it makes them think "Oh, cool!"--would recognize the story name.