Mostly it's due to the fingering, which you'll notice only makes use of fingers 1, 2, and 4 on each hand. These are the more powerful fingers, which reflects the quasi-accented nature of these notes in the line. If you played it with one hand you'd likely be using 3 and 5 a lot, and 5 for sure is a weaker finger for most pianists.
I would also say it's being used just to keep the activity more symmetrical between the hands. Those are the only staffs on the score at that point; there's nothing going on, so there's no point in letting the right hand do all the work while the left takes a break. Most fingerings are editorial (especially in academic arrangements), and I've seen this behavior in the edition I have of Bach's WTC I Bb Major Prelude as well.