Well, the "official" successor to MP3 is AAC, and the successor to that is HE-AAC. And these
are in widespread use, e.g. in the iPod or in 3G mobile phones.
Usually the *users* of standards don't want them to change, because they want to support a
potentially large installed base for a long time. This is especially true in consumer
electronics. (Take MPEG-2 video as example. This was finalized in 1995 and is still in
widespread use.) The situation is somewhat different in the computer world. You can always
download the latest and greatest codecs, but even this may be annoying sometimes.