Benefits of having comments next to selftext instead of beneath it:
Lines of text not as wide, so the user's eyes don't have to jump the whole width of the tablet when going from the end of one line to the next.
Currently, the post text goes out of sight when the user scrolls down to read the comments. If it was in its own pane, it would not have to scroll and so could be visible regardless of how far down the comments section the user would be.
The post text doesn't usually receive any touch interaction from the user, but the comments do. Moving the comments to only be in the right half of the screen would mean less hand movement for the user, as their hand could stay on one side of the screen. This would be especially helpful for large (think 12") tablets where their hand would otherwise have to do quite a lot of movement to interact with different parts of the comment UI. Also, if the left selftext pane wasn't sensitive, the user could physically hold the tablet by that part of the screen.
Benefits of having comments next to selftext instead of beneath it: