well you cannot upgrade the capacity infinitely. There are only so many simultaneous connections possible at a time to a tower, and there is a limit to how many towers can be built in an area. If everyone is using their mobile connections as a fixed line (have already paid for the data, might as well use it all the time, so I get my money's worth) then there simply isn't enough space in the air for most people to use their mobiles at all.
Correct me if im wrong, or someone dig up the numbers or explain better.