One of the most serious challenges to providing low-cost cellular service in rural areas is the lack of available cellular spectrum. Just about everywhere in the world, all of the spectrum is already locked up by incumbent carriers. So, you might ask, if the spectrum is already held, why don't the people living under it have service? The problem is one of granularity.
Rural areas have lower population density and less infrastructure than urban areas. You need taller towers to get greater range. Your cell sites might not have grid power. The best sites may not be near paved roads. These factors make rural areas more expensive to serve. As the same time, perversely, the people who live in these rural areas have less income, and there are a lot less of them. So if you are a cellular carrier with licenses in both rural and urban areas, you have good motives to concentrate on urban service and ignore the rural areas.
Basic physics shows us that urban and rural areas might require different technical approaches. Basic demographics shows us that expectations of profitability are much lower in rural areas than in urban areas. So how do regulators deal with that? They make it nearly impossible to get a cellular license in a rural area without having to get a license in an urban area at the same time. No, that's not supposed to make sense, but it is true nearly everywhere in the world.
Here in the US, the FCC auctioned most cellular licenses by "metropolitan statistical area" (MSA) or "rural statistical area" (RSA). Despite those promising names, more often than not an MSA or RSA is just a county or group of counties. (Here's the map in PDF.) That's why I can't get a license for rural Solano County, California, which is mostly sheep pasture and marshes, without getting licenses for several cities totaling nearly 500,000 people at the same time. That's why I can't get a license for Gerlach, Nevada, an isolated town of about 200 people, without getting a license for Reno, a distant city of more than 200,000, in the bargain. What if you want to serve Gerlach but can't afford a license for Reno? TFB (too ... bad). No license for you!
It's bad enough to do business that way in the US, where even the country folk are affluent by world standards, but in developing countries, where the urban-rural disparity is even greater, most licenses are national. For example, if you want to provide cellular service anywhere in Kenya, you probably need a license for Nairobi. And since median income in Nairobi is around US$160/mo and the median income in the coutryside is less than US$30/mo, you can imagine what that does for the prospects of a small rural carrier ever happening.
If you wanted a licensing system to discourage rural service, it would be hard to design a more effective spectrum allocation policy. Some countries are making noises about changing these policies soon. Let's hope.