I assume you mean the added flap arrangement, as opposed to a simple spade rudder.
TLDR 99% of the time, these are doing nearly nothing, so you want the smallest, lowest drag arrangement when doing nothing. That translates to a very high max lift coefficient arrangement, even if the L/D isn't all that great at high lift.
The reason is these have a significantly higher maximum coefficient of lift, making them smaller for a given amount of force. This is very desirable since 99% of the time they aren't doing much and being smaller, these have less drag then.
One of the best sources detailing the performance of this type mechanical arrangement (which is different from aircraft flap actuation) is probably the BoatDesign.net/hydrodynamics thread on the wing-sail designs used Americas Cup's boats and a few other ultra high performance sailboats. Tom Speer is a retired Boeing engineer who has been working on the last three or four generations of these boat's wings, and has posted analyses of their geometry and performance. The images below are from one of Tom's post's via a bunch of cut and pastes. The entire tread is here - Understanding Wing Technology
See also Tom's homepage, under rigid wingsail rigs. - http://www.tspeer.com/