During the French Revolution, peasants and others not represented by the other estates sat in the left wing (in contrast to the nobility who sat in the right wing). To this day, the left wing continues to represent minorities and the common people.
The left-wing is fighting oppression of the lower classes.
During the French Revolution, the nobility sat in the right wing (in contrast to the common people who sat in the left wing). To this day, the right wing continues to represent the rich and the powerful.
The right wing supports the status quo by oppressing the lower classes.