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}.

[glossary]