I think the reason is the unbalance it might create. SG jumps are ok, because it's much more difficult to get airborn, then attack your opponents. I think they even made the SG switch away time longer for that reason. Rox jumping kinda is like having self-damaging jump boots, from which you can rain fire on your opponents. This is all I did with the battle suit, and fun as it may be, it was quite devistating. Enough armor, and you don't need something killing your self-damage. Switch away time from rox could also lead to some nasty other air weapons, but that would all depend on the weapons. Now, I'd say rox jump to air LtG would be nasty, but in 2k7 the LtG is a relic of the past. As much as I'd like to pull rox jump combos, I think that could have a little too much "wtf" factor for noobs.
More than anything I want balanced weapons, and an aspect like that is something I can't determine as balanced until I know what the deal is with the rest of the weapons. With-holding vote.