Because a scientific theory is based on a lot of physical evidence gathered from experiments.
Work Step by Step
To take an extreme example, suppose the planet were so small that the two cities were 1/4 of the way around the world from one another. When the sun is overhead at Syene, it is at the horizon in Alexandria, and the pillar casts a very long shadow. In other words, on a smaller planet the angle between the sun's rays and the pillar would be larger in Alexandria.