Security: Exchanging usual and widely used formats is less conspicious than marginal dataflow. MP3 files are plausible carrier for confidential communication.
Convenience: Obviously, unsuspicious carrier data are more extensive than the hidden data. Larger carriers simplify the embedding of large hidden messages.
Feasibility: The Information loss during the encoding process creates a certain space of unpredictable noise which can be exploited to carry the hidden information.
MP3 is a promising cover format for steganographic data.