Why nightfall happen?

Answer 1

When the Earth rotates during the day, half of it will be pointing to the sun which creates morning. When it rotates away from the sun, it creates nightfall. This is why Japan is at least 12 hours behind from Montreal. When it's morning there, it's nightime here.

Answer 2

Nightfall happens because as the Earth rotates on its axis, different parts of the planet are either facing towards or away from the sun. When a specific location on Earth is facing away from the sun, it experiences darkness and nightfall occurs.