I don't know what the reason for this is, but I inmagine it has something to do with the fact that if you have 6 cards on the field that are darkness or death then with six nightfall's you would be doing 36 extra damage, and with 6 eclipse cards that would be 72 extra damage, with extremly hard cards to kill. I think if you have one nightfall out and one eclipse out, it would be fine if they stacked. Mainly, there's no weakness to it if it stacks. Blessings can stack because it only effects one card, that's it's weakness, hourglass, if you play wrong, can kill you because if you use them to early in a stall deck you won't have enough cards to wait it out, and the empathetic bond card although it is reliant on creatures as well, doesn't make them harder to kill. So, in short, it would be to strong, and the developer states that nightfall not stacking is intended on the FAQ page.
