[markfenton]

I've no figures to back it up, but my initial thought is that the simple cost of the OS isn't the deciding factor. For a PC people expect the Windows name, to see Windows on it, and for all their Windows software to work. That leaves using Linux as an unseen cost to the supplier. For a phone most people probably don't see an OS (with the probable exception of Apple), and don't have legacy software. They just see a phone they want that does shiny things. Therefore the OS chosen might not have such a hidden cost making the fact that Android is free a bigger advantage.