[Rochus]

In Smalltalk-80 they're on the left side, see e.g. https://github.com/rochus-keller/Smalltalk

As is known, Jobs was inspired by the Smalltalk GUI.

[kps]

I recall reading that the reason is that the mouse pointer is more likely to already be near the left of the window, because more of the text is on the left, especially in a programming language.