I guessing you been living under a rock.. Because my Truck CD Player plays AAC, PS3, Xbox360, PSP, DSi, 3DS, my wife and daughter's Android phone all play AAC.. The list can go on... Google is your friend....
notices something. Most of those devices are a lot newer. Like I said my car from 2004 can play MP3 but not AAC files. Things from back then it was iffy at best if it it could play AAC.
Mp3 plays on everything. AAC does not. MP3 is supported on older devices. AAC is not.
AAC is just not as widely support as MP3 end of story.
Top it off Mp3 is smaller at the same bit rate
http://ipod.about.com/od/introductiontoitunes/a/sound_qual_test.htm
At a certain point you really are not gaining anything. Amazon VBR is a great way to give great quality sound for a smaller file size.