The original BSD machines were set up as developer machines in that the highest priority went to the processes that used them least. Therefore an app that got 3 key strokes a second had higher priority than one that was moving a file. System V, ancestor of Solaris and more the model of linux in a lot of ways gives higher priority to the tasks that use the system more, so a file transfer would have a higher priority than somebody banging on emacs. It was more of a server model. The BSD was developed earlier and multitasked well enough that it was also a better server than most else around with equivilent hardware. Sys V was made for file serving.
'Tis the way I remember it, anyway...