Comment by RickS

I wonder if it's righthanded mouse users? I'm using a vertical ergo mouse beneath my monitor, and your comment made me realize that it's quite similar to if I just.. reached for the scrollbar and pulled it. As opposed to having to cross the whole screen, metaphorically.

There's also the distraction factor. Maybe having the bar moving on the left edge competes with moving from line to line, and the general anchoring edge of the F shaped reading pattern.

Total speculation on my part.