Mp3 (as well as other waveform sound formats) and MIDI are in principle different forms of sound representation. MIDI contains commands, and MP3 is compressed waveform. That's why the task of MP3 to MIDI conversion is much more difficult than, say MP3 to Wave or others.
Here I found one of the most clear description of this process I have seen:
http://www.mp3-to-midi.net/mp3-to-midi.html