Lying on your back to give birth closes down your pelvis and almost always increases pain. So why is this the most commonly used position for birth in the Western world today?

Because of King Louis XIV & Dr. Françios Mauriceau.

At a time when only women attended births, it's reported that King Louis XIV insisted on watching his offspring be born. He didn't like the obstructed view caused by a birthing stool (most common position at the time), so he insisted his wife or mistress give birth on her back with her legs in stirrups.

Once word got out that this is how royalty gave birth, the new position began to gain popularity.

At the same time, French doctor Mauriceau, who believed pregnancy to be pathological, thought women would breathe more easily and have reduced pain when lying in bed for birth. His view of pregnancy as a disease had a hand in the change from midwife-attended birth to the primarily surgeon-attended birth that we see in much of the world today.

Though the lithotomy position is usually the worst option for the woman giving birth, it is still the easiest & most comfortable for those attending the birth. The doctor/midwife/nurse can simply sit on a stool and have a perfect visual, which is a good part of why it is still used today.

Upright positions including squatting, sitting, standing, and hands & knees all allow the pelvis to open and let gravity assist in the baby being born.