Hi Hugo,
Mainly because of mp3's low quality encoding.
anyway, DJ's may broadcast in their favorite format, including MP3. It's just a personal decision to cast in AAC.
Cheers.
Of course it is your decision, but if nobody can hear your radio, you should think about.
Most of the Internet Receivers can't encode AAC.
Even my favorite pc webradio programm (Magix Webradio) can not receiver your station.