Population density is misleading, since it includes places nobody lives like the Mojave desert, Rocky Mountains, and South Dakota.
The Eastern Seaboard of the US has roughly the same population density as South Korea, yet South Koreans get 100Mbit internet for $30/month.
Knowing that most larger ISP's have pocketed record profits each year since about 2007/2008, while I'm sure some money has gone in to advancing the infrastructure, I would say that there was a sort of "gentleman's agreement" that consumers understood that ISP's would invest money beyond the previous records of profit in previous years--which you would think a good company would do--into the infrastructure. Which hasn't really happened. That excess has just been pocketed. Net access has improved slightly over what was available ~10 years ago, with fiber optic connections being available to some of the population, but to be honest, there are a lot of locations that just can't get those higher speeds. The town I grew up in--a small-ish town in northern California--is one example. Only recently--like within the last 3-5 years--has the DSL access speeds gone beyond a 3mbps connection because the phone lines laid down were not able to support the bandwidth. Of course larger cities is a nice place to start as far as improving, but when you have 100mbps access in some areas, and others just a hair beyond dial-up, there's a huge disparity in access. I'm not saying that everyone in that area necessarily WANTS 100mbps connections, but to just now have beyond a 3mbps connection even possible in the past 3 years, it's surprising.