That does not explain why Sweden and Finland have way better coverage while having lower population density. The total area should not matter, only the population density.
The US has a lot of suburban sprawl, so population densities are lower in the places where people actually live. (Still I don't believe this is the real reason.)
True, that could explain why Internet over fiber is less common, but does not explain the poor and expensive cell phone coverage. We have good coverage in most of Sweden, even where virtually nobody lives.
Skipping having 3-4 incompatible networks probably helped a lot (I'm not familiar with the details, but I think at one point the U.S. had Analog, GSM, CDMA and iDEN networks in widespread use).
Greater regulatory uniformity also probably helps a lot.
And then there are things like the New York metropolitan area having 3 times the population of Sweden, while many other states are many times the size of Sweden (so a U.S. national carrier needs to specialize in both serving much higher and much lower density than a Swedish national carrier).
I'm not insisting that the wildly varying density makes the actual provision of service particularly harder, but combine it with many different regulators and no strong overall regulator and it isn't surprising things are a mess.
Also the 1990s 'PCS' auction which created hundreds of teeny-tiny local cellular companies. It took a long time for the big providers to stitch a nationwide network together.
Personally I believe the size has little to do with it. The other factors you mentioned are way more important. The regulation has been handled terribly in the US.
