I have recently researched this topic on the structural changes of the soybean industry since the early 1960's when soybeans really began to be a major protein source. Over the many years, I have found that legislation has a major impact to the industry. Naturally, supply and demand causes shifts in prices and the amount of exports.
Farmers rely on prices to alter their produciton habits.
Since we export every other row of soybeans, the export market is an extremely large player in production activities. As developing countries' population rate increases, and especially their distribution of wealth increases, there will be a greater demand for soybeans. Most of the developing countries rely on the United States and other countries to supply their food, so our export policies as well as their import resrictions have a major role in the shaping of the soybean industry. There are numerous uses for soybean oil and scientists are still finding beneficial uses for soybean oil which will also increase the demand for soybeans. I hope this briefly answers your question.