MP3 is a popular audio format

Security: Exchanging usual and widely used formats is less conspicious than marginal dataflow. MP3 files are plausible carrier for confidential communication.

MP3 files tend to be large

Convenience: Obviously, unsuspicious carrier data are more extensive than the hidden data. Larger carriers simplify the embedding of large hidden messages.

MP3 files are lossy compressed

Feasibility: The Information loss during the encoding process creates a certain space of unpredictable noise which can be exploited to carry the hidden information.

In brief: