It's an R&D and therefore ultimately a cost-benefits decision. Quieting techniques for helicopters aren't as simple as "hush kits" for fixed-wings, because the engines and rotors are a much more integral part of the entire airframe. So, to incorporate these features into a civilian helicopter is a much more involved process than swapping blades on a turboprop or putting hush rings on the exhausts of older airliner engines. Changing the physical properties of the rotors (lift, induced drag, moment of inertia) requires compensating changes in the engine; shorter blades reduces lift requiring higher RPM, more blades increases MOI requiring more torque, etc. Those engine changes will in turn require changes to tail rotor characteristics, and may also require frame and body tweaks, etc etc.
Most helicopters in civilian use today, like the Bell 200-series, Sikorsky S-70 family, Dauphin etc are Vietnam-era designs, almost unchanged save for a few minor tweaks along the way like incremental engine improvements or new avionics systems. They're still being built and flown because they work; take the rigorous proving tests required of any aviation system and the maintenance and inspection requirements of fixed wing aircraft, and add an additional level of caution for the inherent instability and lack of fail-safes in the design of a rotary-wing aircraft, and you'll have some idea of what it takes to get a new helicopter vetted for GA use. If the helicopter's engine cuts out or throws a prop blade, you can't just glide down to the ground the same way as a fixed wing. So, any changes you make to a basic, proven design would be subject to extreme skepticism and testing by both aviation regulators and owners, to say nothing of starting from scratch. This decreases initial sales of a new model in the civilian market, increasing the payback period of your R&D to develop this new design.
Second, in part due to this hesitance by GA to adopt new chopper designs, the funding availability to research and develop any new civilian helicopter design is dwarfed by military interests. As mentioned, practically every design you see in the skies today has a military pedigree and was designed primarily for that purpose, then civilianized for GA use (a notable exception being the Eurocopter Dauphin 2 which was established in the private and law enforcement sectors before militaries took a look). As a result, a lot of the newest stealth technologies available for helicopters, including noise reduction, are heavily classified. For instance, the modifications to the UH-60s used in the Bin Laden raid; civilians really only know that stealth Blackhawks exist, as what they look like and their performance/noise characteristics are all still top secret. That hasn't stopped the human imagination; artist impressions including the concept art for Zero Dark Thirty abound.
In addition, the civilian quiet-copter technologies that aren't classified, like Eurocopter's "Blue Edge" blade design, are mostly protected by international patent. Bell or Sikorsky can't just reverse-engineer the Blue Edge blade and stick it on their choppers. An independently-developed design must be different enough to be patentable, which probably means it will be more complex, and complexity is a bad word in aviation because it means increased probability of system failure or operator error. The other avenues would be to either license Blue Edge from Eurocopter (increasing the cost of a Bell or Sikorsky Blue Edge-equipped model) or wait 15-20 years until the patent expires (further delaying widespread adoption of the technology beyond one brand).
