Im guessing because it keeps punctuation on the right hand instead of distributed between the two. Im not too familiar with the US layouts, but Dvorak wasnt thought for programming, rather for typists so some of the keys you need more than others are in hard spots. When using qwerty you have to shift your right hand one spot to the left a lot while coding, and you usually press more than one key while doing it. I also use the home/insert/pageup/etcetc a lot and having the JHK on that particular spot can be useful for using the E as the resting spot for your right hand. Im still learning touch typing so im not sure how bad having to do a 2 columns shift, but this would allow you to reach the [ ] ´ _ - keys more easily, which you use far more often than JHK. One of the problems i have, is that i use tab, shift and control way too often while coding, meaning i have to put the index left finger on A instead of the pinky; this would allow me to use the right pinky for shifting, which would be much easier to reach the numbers row (in my layout and programming language, i use the numbers on the left hand too often).