I don't think Nogizaka Haruka was too successful initially. Rather, I would say that the dropoffs between each season and the OVAs are in line with the franchise decay you see in a lot of other shows that were popular a couple of years ago like Zero no Tsukaima. Otaku have short attention span and Nogizaka Haruka wasn't exactly a popular series like, say, Shakugan no Shana was (and even that had mediocre sales for its final season compared to its first and second).
I personally think that harems have changed. I don't have evidence to back it up, but I feel like they sell and are produced less than in the mid-2000's and that shows like Infinite Stratos, Haganai, and Oreimo are anomalies that mask how much the harem/action or even just the regular harem have fallen out of favor. There will still be big harem hits, of course. The second seasons of Haganai, Oreimo, and Railgun will probably be really successful. I just think that the successful harem shows are, for the most part, successful because of the light novel industry and that anime producers are more conservative about making harems that aren't successful light novels already.
It kinda feels like harems have been divided into two categories: low-selling, poorly-produced anime intending only to promote a light novel or a visual novel (Campione, Hoshikaka, OreTsuba, Da Capo III probably) or popular, more ambitious series intending to create or bolster their franchise (Oreimo, Sword Art Online, Horizon). Maybe it's just because I started watching anime in 2006, so I don't have good perspective and it has always been like this. I don't really have evidence to back up all of this speculation either.
In this case, though, I would say that Nogizaka Haruka's sales are because of a changing demographic. I just think people lost interest.
Edit: I know the two categories sound almost the same! It makes sense in my head though. :<