How did the ancient cultures of mesopotamia and egypt develop into successful civilizations?

The success of both would be strongly contributed to by a great deal of luck to begin with. This luck pertains to having the right conditions under which to produce the food and water necessary to support large civilizations and communities. The power and force to gain large areas of land to be drawn into the civilization. The power and force to keep control of neighboring captured and incorporated lands. The dedication of the people, normally gained by having the power and force to provide them with safe and contented lives without worry of invasion and destruction. Without such power and growing force (military) the civilization would have been a single community or oasis in the desert.

Seeing that the fledgling civilization would survive, there was a need for accounting for all the food and supplies collected from the outer regions so they invented writing and a written record to keep track with. This forced organization and removed a great deal of chance related to luck and thus yielded a stronger and more powerful nation. With which more land could be conquered and absorbed into the realm of the nation