Good question. I believe it has to do with response time. I remember years ago a Saab technician told me about a problem he had been chasing.
It turns out that Saab uses plastic fuel line between the distributor and the injector. Two lines had been remade with a common line used for such. The black line used was common to VW and Volvo. The clear lines used on Saab and BMW were half again as big in diameter. He fixed the problem by putting the lines back to original and the factory rep that helped with the diagnosis said that the diameter of the lines changed the quantity of fuel in the line which changed the ability of the fuel to respond to sudden changes in flow rate.
I would also presume that the design theory of the differential pressure valves internal to the distributor would be changed by the common rail.
Bosch Master, ASE Master, L1
33 years MB technician