Because your nostrils split their workload. Throughout the day, they each take breaks in a process of alternating congestion and decongestion called the nasal cycle.
At a given moment, if you’re breathing through your nose, the lion’s share of the air is going in and out of one nostril, with a much smaller amount passing through the other. Every few hours, your autonomic nervous system, which takes care of your heart rate, digestion and other things you don’t consciously control, switches things up and your other nostril does all the heavy lifting for a little while.
The opening and closing of the two passages is done by swelling and deflating erectile tissue - the same stuff that’s at work when your reproductive organs are aroused - up in your nose. The nasal cycle is going on all the time, but when you’re sick and really congested, the extra mucous often makes the nostril that’s on break feel much more backed up.
There are at least two good reasons why nasal cycling happens: And Mental Floss explains why…)