To find our answer, we go back to the Industrial Revolution (1760-1840s) when most people lived in urban centers of cities. In the northern hemisphere, the prevailing winds tend to be the westerlies (winds that blow from west to east). These prevailing winds blew the pollution from the factories to east resulting in poor air quality and heavy "dust" (sedimentation from burning coal) on the east side of cities. As most people would prefer cleaner air quality and much less "dust", those with the means would settle on the west side of cities leaving the east side for the less fortunate.
The next time you are in an older city, check to see if this pattern holds true.