This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
Well, maybe because they started to design and develop their systems
rather early, e.g. in 1990. As Layer-2 is identical with MUSICAM, it has
been available since summer of 1990, at latest. In that year, Layer-3
development started and could be successfully finished at the end of 1991.
So, for a certain time, vendors could only exploit the already existing part
of the new MPEG standard.
Now the situation has changed. All Layers are available, the standard is
completed, and new systems may capitalize on the full features of MPEG
audio.
 
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