Mostly because the air in your nose goes straight through one of you nostrils or turns into the other nostril and voila you breath out of one side of your nose.
Answer: Our nostrils are basically one nose just with something in between them.
Explanation: Our nostrils are separated by a septum, in effect giving us two noses. Most of the time, one nostril allows less air to pass through than the other, with the nasal flow switching every few hours. The slower airflow is caused by the tissue inside swelling with increased blood flow.
