There are two main reasons why Mesopotamia was invaded so many times in ancient times.
First, there is the fact that Mesopotamia was a relatively abundant land. In those days, the region was more congenial to agriculture than it is today. It was, for this reason, called the “Fertile Crescent.” When an area has great riches, it is likely to attract outside forces that would like to invade it.
Second, there is the geography of the region. Some regions are relatively safe from invasion. Britain is a good example of this as it is protected from invasion by ocean. No invasion has even been seriously attempted since 1066. By contrast, Mesopotamia was not protected. It had no mountain ranges to wall it off from other areas. Its rivers were not a particularly important obstacle. This meant that invaders could easily enter the region.
Thus, Mesopotamia was both a tempting mark and an easy one. It was rich and it was relatively unguarded. Therefore, it is not surprising that it was invaded so often.