Radio is one of several resources used by manufacturers, retailers, etc. to market their goods and services. Same thing with religion... it's not a matter of being the "best way", but one of the ways to reach potential and current followers. Radio has the advantage of being available to just about anyone, anywhere. It is optional, in that one can choose to twist the dial or turn it off entirely. With some sort of programming on just about every channel, there's something for almost anyone, and for the few who don't find what they want, there's Sirius, internet, CDs, cassettes, and eight-track.
Other than power and location, there's nothing inherently special about a specific FM frequency, so I'm not sure why this appears to be such an obsession for some, but I imagine it beats Saturday night television.
I think the discussion of religion doesn't belong here. Virtually all religions believe that theirs is the "one true church".