The rig is probably a survival from a much earlier period. Gaff rigs were preferable to large triangular sails in an era in which even battens would not prevent a sail from tearing under stress. The huge triangular sails of racing yachts are only possible today because of special fabrics which have only been avaiable for less than half a century.
The image above is of America
, the racing yacht which won Prince Albert's cup in 1851, and for which the America's Cup has been named. Note that it uses a gaff sail with a drabbler above that sail, and a gaff foresail rather than any large jib. You can also see this in the racing yacht in the background. Sails which were not quadrangular simply could not stand heavy stresses in the days when sails were made of linen canvas or cotton canvas.