I'm not going to lie and say that the regressiveness and people's unwillingness to change that mp3 represents have nothing to do with it, but another issue is wastefulness. It is a comparatively inefficient audio codec. If people are unwilling to do the easiest thing, with added benefits in favor of less waste, that leaves us with poor prospects as a society. I'm not saying an audio codec is particularly important compared to other issues like global warming, but it is what it is.
So if you wanted a deeper answer, there you have it. But the important bit is that mp3 is not a good codec to be using in 2015.
And I never said I prefer aac, only that it's better than mp3. Pretty much any modern, lossy audio codec is better than mp3. Vorbis and AAC just happen to be mainstream enough to be viable choices. Opus is on the horizon and beats them both.
DRM can be placed on a lot of things. AAC just happened to support it as a feature. I'm the farthest thing from an apple fanboy, in fact I hate apple's practices. Either way, DRM audio files seem to be a thing of the past.