Sure, NHL isn't as popular in America as the "Big 3". I mean, baseball is an American pastime (though losing popularity) and football is just as dear to many people's hearts. I honestly believe the climate has to do with it. I'm from Canada, and though many sports are popular here, it's safe to say we eat, breathe, and sleep hockey. But think about states in America like Minnesota where it is rather cold. I'd say hockey is fairly popular there. Now think about places like California (before the Kings swept clean last season) and Arizona. Hockey isn't nearly as important as basketball, baseball or football. Also you might consider the locations of the Original 6.
People are interested in sports they can identify with. Growing up in northern Alberta, Canada I'm not so inclined to follow beach volleyball. But I'll follow hockey religiously.
The NHL can't do much more to promote hockey in America. Perhaps ESPN could. I think southern and Midwest states just need to be exposed to it a little more. Most sports channels are going to air NFL or college football games all day.
I'd also like to mention that hockey fans in the US have no problem filling out arenas with 20,000 people a night. So its popularity is still on the rise.