Explain why acid rain is unevenly distributed across the world.

Acid rain is rain that has a slightly more acidic pH level than normal.  Pure water has a pH level of 7.  Rain is generally slightly acidic to begin with, so "acid rain" is rain that tends to have a pH of 5.0 - 5.5.  The acid rain itself can be created through a combination of natural and man-made sources.  For example, volcanic eruptions give off elements that help to produce acid rain.  Human sources come from car exhausts and various fossil fuel burning activities.  
Both of the above natural and man-made sources of acid rain are not evenly distributed across the globe.  Population densities vary across continent to continent.  Additionally, natural soil compositions differ as well.  Lastly, prevailing winds can carry acid rain and its components far away from their source regions.  All of those reasons combined explain why acid rain is not evenly distributed across the globe.  In the United States for example, acid rain tends to occur most in the Northeastern United States.  This is because of the high population density, the number of large cities (with their power plants), and the winds tend to carry Midwest pollution to the Northeastern United States.