In the US, the big thing is the proprietary magazine used by the MR762/308. Those magazines are not cheap. This deters a lot of people based on price point. HK stuff is also proprietary, that does not go with the general firearms hobbyists that need to switch parts out all the time and look for customization.
Like someone has said before, there had been some pretty strong anti-HK sentiment cultivated over the years. HK customer service and their corporate ineptness in the past were to be blamed for it, but the German government's muddling of arms sale issue and export control is a large of of the problem that hinders HK business, especially there is a segment of leftwinged German wants to see HK gone.
Finally, it has been a pretty bitter pill for some of the American manufacturers to swallow when HK blindsided them in getting the special forces adopted the HK 416 ( and the XM-8 earlier ). It is a blow because the top tier guys bought in the HK416 and MP7, and basically discredited the SCAR trial and the M4. The entire special forces small arm market in the world outside of the US is also cornered by HK416. The general public eats up the sour grape because they can't get the forbidden grape. If people can't get it, they talk #### about it because it hurts the feelings when they can't get what the special forces use. So we have this paradox when people will buy whatever the special forces use and build their "socom M4 clone", "MK12 clone", "CQBR clone", but when it comes to HK they get into denial. They hope the HK would have never be adopted by the US special forces so they would not have to deal with the forbidden grape.
