The frequency allocations are indeed determined by a regulatory agency like the FCC or the ITU.
An important factor in this process is the propagation characteristics of the band.
100 - 150 MHz are popular and used for all kinds of public services because they have a high bandwidth, but still retain the ability to 'bend' around obstacles, greatly increasing their coverage in broadcast situations with a single transmitter serving hundreds ( thousands ) of square miles.
Cellular companies were granted 800 MHz bands because they have , again , high bandwidth, but are extremely limited to line-of-site and therefore are less desirable. Cellular companies used these frequencies in a cellular pattern, with each cell covering much less ( say 25 - 50 ) square miles.