@TenthJustice: I can agree fully with your comment, but I'm not arguing against the use of stealth per se. Rather, considering the orcs moved through the darkness of caves without light (at least, the horde that surrounds them earlier in Moria so does), darkness seems less of a "cover" to their eyes than daylight, when they are less likely to even be "outside" to see anything.

So though I have +1'd this (as it moves toward a plausible answer), a source reference from Tolkien's lore that the orcs would have needed more time to "get out" of Moria and swarm the hills because the exit via Khazad-dum had no other exits near it (hence why they are not swarming almost immediately) and some reference that orcs do not see much better at night than men (though they live in the dark) would boost your answer significantly. Or any other Tolkien references that might be applicable.

Regarding your 1st reason, the orcs were hot on their trail already, and though the bridge of Khazad-dum had been destroyed, it seems that other ways to exit and swarm the hills would have been not far. So I'm not entirely convinced the first reference had no relation to orc movement at night. Regarding the 2nd, the new kind of orc, made by Saruman, moves easily/willingly in daylight, but Aragorn does not yet know this. Granting that orcs can move in daylight, normally they emerge at night (and presumably can see well in the dark), so I'm not sure your answer offers enough in its argument.