Spectrum
The radio frequencies available — a finite natural resource that states auction off for billions.
Low frequencies travel far and penetrate walls but carry less data. High frequencies are the reverse. That alone explains why rural coverage is built on low bands while urban capacity comes from high ones, and why no single band suffices for a working network.
Allocation is coordinated globally through the ITU and nationally by regulators. The auctions have been enormous transactions; the 3G licences around the year 2000 cost European operators over a hundred billion euros in total and contributed to sinking several of them. Some bands are unlicensed, which is why wifi and Bluetooth can be used freely.