I'm not an electrical engineer, but I think that one difference can be in the required drive voltage and power for the bus. Remember that the GPU needs to drive

I'm not an electrical engineer, but I think that one difference can be in the required drive voltage and power for the bus. Remember that the GPU needs to drive the bus, i.e. it must apply sufficient voltage to make the lines reach the required voltages, and do so quickly. This can become easier if you know the distance is smaller; the driver can be faster since the losses in the line will be smaller too (and less risk of it picking up interference). It's probably a differential drive which makes things more complicated in the details, but I think in general the above is true anyway.