Was it simply because there's so many Sherlock Holmes ripoffs involving a "smug genius teams up with cop/FBI agent/etc. to solves crimes" that you would think that it really needed something special to stand out? The premise (https://www.quora.com/Why-was-the-last-season-of-The-Mentalist-so-unsuccessful) is pretty much an exact copy of Psych, which premiered earlier, albeit with less fun and humor.

It's not underwatched by me. It's still one of my favorite shows and I still watch it every week.