As far as I know, .US is not that restricted. There are Nexus requirements, but you don't have to live there, you just have to do business in the US. And if you look at it in terms of end users, whose going to want to buy a .us? Someone who either lives there or does business there. So generally speaking I don't think the restrictions are that tight that it will effect the market to an extreme degree. The major factor is just it's lack of popularity and usage. Most people in the US have never heard of .us. To be honest I think its a bit silly to introduce a ccTLD for a country that invented the .com