* US population density: 35/km2
* Finland population density: 18/km2
It's not about the size of the country, it's because in Europe the infrastructure owners are required to let others in [1]. In US the operators are natural monopolies with very little incentives for competition.
Finland has a higher proportion of urban residents than the US. The US is also known for its huge number of small and mid-sized cities and population spread all over the country, whereas (proportionately) more of Finland is essentially empty.
That may be true but doesn't really factor in to the discussion of coverage and competition: even if essentially empty, Finland is still covered. Coverage maps for the two biggest operators:
[1] https://en.wikipedia.org/wiki/Mobile_virtual_network_operato...