a) Storm hadn't arrived yet,
b) Storm couldn't make it as dark as it was with just clouds

It wasn't even dusk when he started moving the bridge, so the whole day night thing was probobly just a device for coolness, moving the bridge looks better in full light, a "Last Stand" fight looks better under flood lights.

Another of the many holes I noticed...if Magneto can pick up the GG bridge AND keep it from collapsing under its own weight with no supports then surely he could have picked up that prison building, which was most likely built of concrete reinforced with steel beams, and dropped it in the Bay, drowning said annoying kid.