Why do some airplanes allow separate right and left rudder pedal adjustment?
As with any question along these lines—"Why did the designers do X"—a definitive answer is very difficult absent specific documentation, which is rare.

I am not familiar with the Dornier 228, but I am familiar with the King Air B200, which is a very similar class of aircraft. Beechcraft also designed the King Air with individual pedal adjustment.

In my examination of that system, I believe the primary reason is simplicity: the individual adjustment system on each pedal is simple and lightweight, whereas a system to adjust both pedals simultaneously would likely be heavier and more complex. Weight and complexity are both undesirable characteristics in aircraft design.

See this excerpt from the King Air B200 Illustrated Parts Catalogue (IPC) depicting the left hand side rudder pedal assemblies:

 
Source: KA B200 IPC 27-20-00-06

I have highlighted the rudder pedal adjustment lever assemblies—B and C—in light blue. I have highlighted the adjustment pin—No. 375—in green. (Note that the IPC incorrectly depicts the two adjustment levers in reversed locations; see the aircraft photo below.)

The figure clearly shows the simplicity of the design and the way the adjustment relates to the overall rudder pedal control system. The adjustment lever is a simple assembly of stamped metal and hardware, light and cheap to manufacture. The system is simple and intuitive to use and requires little or no rigging.

This is a photo of the left hand side rudder pedal assembly in an actual King Air B200; adjustment is accomplished by depressing the lever with the outer edge of one's shoe and moving the pedal forward or aft:



Any design allowing simultaneous dual adjustment through a single adjustment action would likely have resulted in some combination of increased complexity, expense, weight, or maintenance requirements.

Moreover the system, as designed, allows for separate adjustment, which could be helpful for some individuals or situations.