It was largely the English who were the original big fans of black tea. They first imported large quantities of black tea from China until they started their own tea production in the colonies of India and Sri Lanka (then Ceylon) in the mid- to late 19th century. Before that, China was actually the only country in the world that produced black tea.
In addition to the English discovering and cultivating the Indian type of tea plant, Camellia sinensis assamica, they also managed to obtain (or perhaps rather steal) Chinese tea plants and start the first production of tea outside of China and Japan. The British botanist and adventurer Robert Fortune managed to get into the then fairly closed Chinese hinterland and under what seems to have been very adventurous conditions, he managed to acquire tea plants which he, with the help of the English state, planted in India. The experiments with the plants and the production were often very demanding, but after many failures, a professional production was developed, and when the business was industrialized around the 1880s, the market also grew globally.
The English developed the production and trade of black teas. In 1886, England imported a whopping 77,000 tons of tea from China, but after their production in India and Sri Lanka got going in earnest, imports from China dropped dramatically and almost ceased later in the 20th century. Due to the shipping advantages and Westerners getting used to the taste, black tea was the way to go, and that is actually the reason why we still mainly drink black tea.
Even in Russia and its neighboring countries, including Mongolia, black tea is important, and the same applies not least to the former British colonies of India and Sri Lanka, where black tea is basically the only thing people drink. In China, hardly any black tea is drunk at all despite its significant production; it is exported. Black tea is also produced in Russia, Turkey and Iran (mainly for domestic consumption), and during the 20th century, African countries, led by Kenya, emerged as major producers of black tea. So far, there are no high-quality varieties, but it is probably only a matter of time.