Its called "Market".... for all i could see, when the simpit market started, the most popular AC was boeing 737.. so all effords were focused on that aircraft, when demand was strong enough to make a point, major sim suppliers started to make them on a comercial level...you have also more real parts available from boeing planes..so its easier to get a few of them, and copy them properly...airbus real parts are really really really really hard to come by .. the market is rotating though, airbus is getting popular by the day, so in my opinion you'll see a hudge development in airbus parts for cockpit building..
The most realistic ones in my opinion are made by www.simparts.de , only a few available though.... FDS with the new line of panels also is a great supplier...
the only thing i do have to criticize to all manufacturers at this point is..can anyone please finally get the color right?!.......................
cheers!
Alex