Why did John Steinbeck decide to write The Pearl?

In order to assist you with this question, and without giving you my personal answer as this appears to be a question requiring a student to deeply think about context and purpose, it's important to consider the themes of this captivating novel.  If one concludes that ultimately Steinbeck's The Pearl is a novella about the consequences of greed and the dangers of extreme wealth, then one can easily draw a logical conclusion of his ultimate purpose for writing.