Because the FAA is not sure if your particular phone emits the same RF signals that the plane's systems use. Your phone probably won't interfere with the plane's navigation system, but the only way to be sure is to have the FAA certify every single device which is probably not going to happen given how often new models come out. The rules have been changed for quite a while now to allow you to use them in flight, that's why onboard WiFi is available now.