Until well into the 20th century, most merchant ships were general cargo vessels, able to carry anything and everything. They had to be prepared to move any goods that needed moving, and usually sailed with mixed cargoes.
Goods came in different shapes and sizes and needed to be handled in different ways. Loading and unloading at ports was time-consuming (it was very slow) and labour-intensive (it required many workers). This added to the cost of shipping and made all goods more expensive.
Economic changes in the 20th century meant changes in sea-borne trade. Mass production of industrial goods increased international trade, as manufactured goods and the raw materials needed to make them were shipped overseas in ever-larger quantities.
After the Second World War, specialist vessels designed for specific cargoes - bulk carriers and container ships - largely replaced the old general ships.