My guess would be climate. 100K ago, humans were in the midst of the ice age and it lasted until about 12,000 years ago, at the start of the Holocene period. The early Holocene was actually warmer than modern temperatures, and it is said that even the Sahara was fertile in this time. As the ice age ended, rain patterns shifted and vegetation flourished. It probably then became possible to remain in one area for an extended period of time without roaming around in search of game and wild food, and you could actually support a larger tribe of people. Once that began to happen, people started learning and developing local culture, language, skill development, etc. and sharing that knowledge. For the same reason we see hubs of specialized labor in modern society (like Silicon Valley, Hollywood, New York financial district, etc), the first communities to develop agriculture were able to exploit those skills to improve techniques over time, and caused places like Mesopotamia and Egypt to develop and thrive over time. It appears as though Mesopotamia developed clay tablets with written language as an early form of contractual agreement rather than as a form of communication or literature.