Because humans are much worse at estimating the relative size of angles than of lengths. See also: http://en.wikipedia.org/wiki/Pie_chart#Use.2C_effectiveness_

Because humans are much worse at estimating the relative size of angles than of lengths. See also: http://en.wikipedia.org/wiki/Pie_chart#Use.2C_effectiveness_and_visual_perception http://en.wikipedia.org/wiki/Pie_chart#Use.2C_effectiveness_... Of course, the very worst are 3D pie charts, as the angles are distorted.