Why does it get dark at night?

Because Earth, on its axis, turns away from the sun. Every time Earth, on its axis turns it becomes day or night.

During the day, daylight floods our environment every which way, with both immediate and reflected daylight coming to us from wherever we can see. Around evening time, the daylight doesn't flood the environment, as it's dull wherever in the sky that there isn't a state of light at, similar to a star, planet, or the Moon.