Final answer:
The Kievan Rus' primarily collapsed due to the Mongol invasion in the 13th century, which caused substantial socio-economic decline and political disruption.
Step-by-step explanation:
The primary reason for the collapse of the Kievan Rus' was the Mongol invasion in the 13th century. This devastating event undermined the region's stability and marked the beginning of the end for the once flourishing confederation of city-states. Despite having a significant trading position and cultural development, Kievan Rus' could not withstand the military onslaught and political disruption brought on by the Mongols. The invasion disrupted governance structures, trade routes, and led to a massive socio-economic decline.