The main reason for "historical inertia" is that, when aircraft radios were first adopted, AM was just about all the current technology would support, and the International Aviation and Communication regulating bodies had to decide on a "common" frequency spectrum and technical standard. Since then, even thought FM has proven itself far superior, it would not be feasible to change over today. ALL aircraft radios and ground communication equipment would have to be changed SIMULTANEOUSLY worldwide. Additionally, the frequency spectrum is completely filled up and probably cannot support a new range exclusively for aviation use. Many countries use differing frequency allotments for some uses. For example, business band radios in the US (~151 Mhz) are not legal in Australia, as that frequency range is allotted for other licensed users.