Why do I only breathe out of one nostril?
Apparently you're not the first person to notice this; in 1895, a German nose specialist called Richard Kayser found that we have tissue called erectile tissue in our noses (yes, it is very similar to the tissue found in a penis). This tissue swells in one nostril and shrinks in the other, creating an open airway via only one nostril. What's more, he found that this is indeed a 'nasal cycle', changing every 2.5 hours or so. Of course, the other nostril isn't completely blocked, just mostly. If you try, you can feel a very light push of air out of the blocked nostril.  

This is controlled by the autonomic nervous system. You can change which nostril is closed and which is open by laying on one side to open the opposite one.  

Interestingly, some researchers think that this is the reason we often switch the sides we lay on during sleep rather regularly, as it is more comfortable to sleep on the side with the blocked nostril downwards.  

As to why we don't breathe through both nostrils simultaneously, I couldn't find anything that explains it.  

Sources:
About 85% of People Only Breathe Out of One Nostril at a Time
Nasal cycle