Why have ringback tones failed in the U.S.?

Just a few years ago, analysts were predicting that ringback tones would take over the mobile music market. Ringtones had been selling successfully so why shouldn't ringback tones--essentially reverse-ringtones, where a song or sound clip is heard by the caller rather than the receiver--achieve the same level of success? Ringback tones are top sellers in Asia and in certain parts of Africa, but why have they never caught fire here? There are multiple reasons why ringback tones never caught in the U.S., including easy access to digital music and the difficulty in marketing ringback tones. Special feature