One factor In the US is definitely frequency allocations. In the US, per FCC rules, there are twelve 6-MHz VHF Channels (Analog channels 2-13) while there are 69 UHF Channels (Analog channels 13-83). Another could be how VHF and UHF propagate, VHF has the distance, aka line of sight advantage due to longer wavelength, while UHF’s higher frequency penetrates obstacles like buildings and trees much more efficiently. Hate to only vite Wikipedia but they have some great (and accurate info), as well as this beautiful Frequency allocation chart, readable high-res PDF here
