Both me and ogbr think it's because of j fierce higher hitbox.
If the opponent messes up, you have an opening with a huge windows to punish, you can do j RH pretty late.
But if the windows of opportunity is short, you need perfect timing for j.rh.
You either hit it too early and you got the j. rh but cannot connect after you land.
Or if you press j.rh too late and the opponent can block.
But for j. fierce even if you hit it slightly earlier, the first active hitbox won't connect, while it will still connect on the way down.
Thus j fierce is easier to combo jumpins
it's some magical pixel calculation the designer did there.