hysterical reasons

from Jargon File (4.4.4, 14 Aug 2003)

hysterical reasons
 n.

   (also hysterical raisins) A variant on the stock phrase "for
   historical reasons", indicating specifically that something must be
   done in some stupid way for backwards compatibility, and moreover that
   the feature it must be compatible with was the result of a bad design
   in the first place. "All IBM PC video adapters have to support MDA
   text mode for hysterical reasons." Compare {bug-for-bug compatible}.

from The Free On-line Dictionary of Computing (8 July 2008)

hysterical reasons

   (Or "hysterical raisins") A variant on the stock phrase "for
   historical reasons", indicating specifically that something
   must be done in some stupid way for backward compatibility,
   and moreover that the feature it must be compatible with was
   the result of a bad design in the first place.  "All IBM PC
   video adaptors have to support MDA text mode for hysterical
   reasons."  Compare {bug-for-bug compatible}.

   [{Jargon File}]