This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
The general belief is that it was chosen because the CD designers wanted to
have a format that could hold Beethoven's ninth symphony. They were trying
to figure out what dimensions to use, and the length of certain performances
There are several different versions of the story. Some say a Polygram (then
part of Philips) artist named Herbert von Karajan wanted his favorite piece
to fit on one disc. Another claims the wife of the Sony chairman wanted
it to hold her favorite symphony. An interview in the July 1992 issue of
_CD-ROM Professional_ reports a Mr. Oga at Sony made the defining request.
(This is almost certainly Norio Ohga, who became President and COO of Sony
in 1982 and has been a high-level executive ever since.)
The "urban legends" web site has some interesting articles for anyone
wishing to puruse the matter further. The relationship of Beethoven's
ninth to the length is noted "believed true" in the alt.folklore.urban FAQ
listing, but no particular variant is endorsed.
Searching the net will reveal any number of "very reliable sources" with
sundry variations on the theme.