Long term historical trend, lots of small tech improvements that add up, like all other tech. Some of it's how antennas are higher gain, which puts more of the energy in the path from one end of a line to the other and wasting less (affecting both cellular and WiFi standards over this period), some is improved compute reducing the cost of routing, but as with the improvements to chips and batteries and PV, the list of things is long and each one only contributes part of it.

EDIT: got the maths very wrong with some other estimates, deleted them.