A couple factors go into why it’s taken so long for AAC to gain traction and become a more widely adopted audio format. For one the format came along after MP3 was well established so it was fighting an uphill battle, and for the longest time the format was incompatible [wouldn’t work] with most operating systems or digital audio players outside of Apple products.

The AAC format was designed by the Apple Corporation in the early 2000’s, and back in those days it was common practice in the computing industry for companies to not carry or support their competitors software. This made AAC playback on PC and other devices more difficult than the already widely used and similar MP3 audio format. Already being established, and it’s ease of use made the MP3 a much more attractive product on non-Mac devices. 

Bottom line MP3 being the favored son of the digital audio file wars came down to a matter of it was there first, and convenience. Meaning that if you were using the AAC format you were limited in who you could share it with and where you could use it.