In the world of yarn, cotton is the 25 million ton, prehistorically domesticated elephant in the room. It is the most widely used fiber in the global textile industry, eclipsing all other fibers combined, and its usage dates back to at least the fifth millennium B.C. (by the Indus Valley Civilization).
The popularity of cotton was really cemented after the invention of the cotton gin in 1793 - a machine that transformed it into a commodity that became the economic backbone of the world's wealthiest economies. And while cotton may no longer be the king crop it once was, it is still easily the most important plant based product to the world of fabrics.
Cotton's primary advantage is its install base; while crops like hemp and bamboo can theoretically be produced much more cheaply than cotton, cotton has an established market position and highly developed agricultural practices to support its production and trade. It is also very soft and very versatile, producing clothing for just about any season or occasion.
China and India currently produce the most cotton on Earth (about 6,500,000 tons per year each), while the United States remains the world's largest exporter, selling over $7.6 billion in cotton annually.