"Why aren't they just straight?"
Neat question. But judging from all the given answers it seems that nobody is really none too sure -- me included.
A lot of the fly boys are using the similar pull-pull system on their rudders, but mainly by emplolying cables, as opposed to using piano wire.
I think the main reason for the V-shaped Ackerman is to counter any glitches or snags as the control surface is moved through its arc. You get a nice, silky-smooth control of the surface if the 'V' end is facing AWAY from the servo. Having it the other way around means you would probably run into all sorts of problems. Those same problems can occur if the Ackerman is straight.
My take is that it's a lot to do with geometry. For instance, if it were straight, you would likely notice one side of the cable would stay nice and taught throughtout its traverse, while the opposite side becomes sloppy (and vice-versa).
An engineer I ain't. All I know is that Mr. Ackerman's invention works.