Why did first gerneration followers of Gautama Buddha chose oral method instead of written verbal method to preserve his teachings?

According to this wikipedia article on Gautama Buddha, > Accounts of his life, discourses, and monastic rules are believed by Buddhists to have been summarized after his death and memorized by his followers. Various collections of teachings attributed to him were passed down by oral tradition and first committed to writing about 400 years later. Why were the teachings of Buddha not preserved in writings by his first generation of followers? In other words, what is the reason for adopting an oral method instead of written verbal method?