When comparing the composition of world trade in the early 20th century to the early 21st century, we find major compositional changes. These include a relative decline in trade in agricultural and primary-products (including raw materials)

How would you explain this in terms of broad historical developments during this period?

The typical composition of world production during this period experienced major changes. Focusing on today's Industrialized Countries (primarily members of the OECD), the industrial-employment composition was focused primarily on agriculture. Most value was in land. The predominant single consumption category was food. Since then, the economies shifted from the agricultural to the manufacturing sectors (continuing trends begun over a century earlier in the industrial revolution). Incomes rose, and consumption shifted in favor of (increasingly affordable) manufactures. Both income and price elasticities were greater in manufactures than in agricultural products. At the same time there was a steady tendency for synthetic (manufactured) inputs to replace agricultural based raw materials and industrial inputs. Hence, trade and of course international trade conformed to overall changes in patterns of world production and consumption.