A physical / technological limits, such as 3G / 4G masts
B number of masts
C number of customers
Even in a perfect market B can't keep up with C until the market is a bit more saturated with smartphones. But in many countries, including the US, phone companies have nasty monopolies and other incentives to not hurry up with building capacity.
So let's stop the hype over 4G and focus on reaching $1 / gigabyte and decent bandwidth during rush hours first.
Also, in some European countries, like The Netherlands, Telco's are actually increasing the cost per gigabyte, blocking Skype and other competitors and reintroducing data caps. They lost a lot of money in the past few years and are trying to recover from that.