Look at the most free market of them all with the least regulation: United States. Here using a mobile phone is WAAAY more expensive than in Europe. My US friends always thinks I'm lying when I tell them how little we pay for data in EU.
In the US nearly everyone has no cost roaming and long distance coast to coast. Having a set phone budget is worth more to me than lower costs. I have friends who have gone for the much cheaper budget plan that only covers their local area: they love it for two months then hit a situation where either it doesn't work or they are get a big bill because they forget they traveled to a roaming area.
As other have pointed out the population density works against us a little too. Having service when I visit remote places is worth a lot too.
The size of the country will play a big role. Many European states are densely populated. Germany, for example, has more than 80mln inhabitants while it is smaller than the state of Montana.
You won't have reception everywhere in Montana but just covering major highways and the interstate system in the US adds a lot of network costs. The barriers to enter are huge whereas you can easily provide coverage for a large part of the population in some EU countries.
You can't go by pure density. Imagine if you transported Mexico City into Antartica. It would be sparsely populated, but highly concentrated. Cellar coverage would be easy.
Are rural areas in Sweden dominated by small, fairly dense towns? In America, even small cities are sprawling because of cars. People will live 10 miles outside of a small town of 3000 people.
