It's true that yaw control is only one axis, so a single (1-axis) control lever, as in 1 pedal, would theoretically be enough. In standard helicopter design, the two pedals in a helicopter also only actuate 1 axis (yaw), and sometimes (usually?) they are even fixed to a bar that rotates around a single center pivot.
While I'm not a helicopter controls designer, I assume the main reasons for that are:
History - Helicopters became airworthy after airplanes had been common for a while. Autogyros date to 1923, and some helicopter development seems to have taken place in the late 1920s to the late 1930s, with the first mass produced helicopter being Sikorsky R-4, in 1944 (https://en.wikipedia.org/wiki/Helicopter#First_practical_rotorcraft). Fixed wing airplanes were a lot earlier, e.g. the Blériot XIII dates to 1908. It was this airplane that was using the yoke + rudder pedals controls, and in airplanes, this type of control was to stay. It's not very far fetched for a new design, to use proven controls that pilots are used to - especially in the early testing phases.
Controls precision - There is a reason we use our hands to write, or to operate machinery. A kettledrum may be operated per pedal, but a violinist usually doesn't stomp on the strings (unless angry?). Most humans are quite capable of doing precise complex movements with their hands, and a lot less with their legs/feet. Feet-operated controls are usually the ones that require a lot of force but are not super delicate, like a brake pedal in an old car without power brake. Using both feet to pivot a single bar for yaw makes precise control much easier.
force symmetry: in case of maneuver loads (say, a +3g turn), in a two pedal design both pedals get the same force, and they cancel each other out. In a single pedal design, whether push-pull or whatever, it's the pilot who needs to counter-act those forces by muscle strength if no control input is desired