Perhaps because it was very profitable? A few things happened about the same time, in the early 19th century, to create that. Cotton turned out to grow well in the American South, which because of its cheap slave labor and large tracts of arable land was able to greatly increase production of it. The Cotton Gin was invented, which created way to efficiently extract the seeds- which before had been a very time consuming laborious process. The new steamboats could easily transport large amounts of it on Southern rivers to ports. Cotton fiber itself worked well in the new large industrial spinning and weaving mills of England and, soon, New England. The other major fibers of the time- wool, linen, and silk- were far harder to grow or raise in large quantities, and/or required more processing, so there was also not much competition for cotton until synthetic fibers came in much later.

The result was a major economic boom for the American South ( which created a major obstacle to the abolition of slavery ). That boom would not likely have lasted, however. Success always breeds imitation, in business, and other countries - like India- also began growing cotton. It has been suggested that, if the South had not had slavery abolished with the Civil War, it would have soon found itself saddled with huge slave workforce and a much smaller market for what they could produce. So, like the oil business, there could have been both a boom and a bust.