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?
I don't know but maybe writing hadn't been invented there yet (or not introduced from elsewhere), or at least it wasn't common.
Wikipedia says,

First appearance of writing in the Indian Subcontinent
The first introduction of writing to the Indian Subcontinent apart from the Bronze Age Indus script, which is undeciphered and may not be an actual script, is mostly identified as the Edicts of Ashoka from c. 250 BCE.

The same is true of Hindu texts, i.e. the earliest records of writing dates from a couple of hundred years after the Buddha,

Sanskrit -- Origin_and_development
The earliest attested Sanskrit texts are religious texts of the Rigveda, from the mid-to-late second millennium BCE. No written records from such an early period survive, if they ever existed.

The article on Brahmi script says,

Brahmi (brāhmī) is the modern name given to one of the oldest writing systems used in South and Central Asia during the final centuries BCE and the early centuries CE. ... The origin of the script is still much debated, with most scholars concurring that Brahmi was derived from or at least influenced by one or more contemporary Semitic scripts, while others favoring the idea of an indigenous origin or connected to the much older and as-yet undeciphered Indus script.

So, who knows, perhaps that kind of writing was introduced via the Alexandrian empire.