I love the old NeXT stuff, and all that they've bequeathed us, but I always wondered why they put the scrollbar on the left side of the window?

NeXT put it on the left to differentiate themselves from Apple, who put it on the right! (Or maybe they just missed Smalltalk-80.)

When NeXT merged into Apple, the UI was harmonized with that of MacOS, and the scrollbar went back to the right. Until it vanished and was largely replaced by multitouch gestures.

Why not?

At least in left-to-right scripts, it makes some sense. You probably look at the left side of the text for orientation rather then the right side - it's where titles and headers are, it's where paragraphs begin etc.

Because it was common in other Windowing environments?

Ahhhh, the good ‘ol days. Love it.