hysterical reasons

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}.