Good question with a lot of possible reasons. The original Cougars did have some teething issues with the rotating barrels seizing up. To a degree the PX4 and the Stoeger Cougar may still suffer a little from that original reputation. The rotating barrel is not something I would personally want to take to say the deserts of Iraq but for most civilian and police work purposes it is excellent. Polarizing style. Personally I like it but it is dramatic and I can certainly see where people will either love it or hate it. Size and weight. Large, wide and heavy for a polymer compared to an M&P or Glock. Personally I think the shape and nicely contoured slide make up for it's girth making it surprisingly easy to carry. Lastly and perhaps most importantly is variety. We the consumer have so many good choices to choose from today that some good models may merely be overlooked. Not sure how hard Beretta markets the PX4 or any of their stuff for that matter but I do know S&W, Springfield, Sig and Ruger spend a ton on marketing and shout loudly from the roof tops when they launch something new.
I wouldn't sweat it's dark horse status. Personally I like having something that is a little unique and isn't found in every other holster.