During World War II, foods that were known to be rich in Vitamin C (e.g. oranges) were almost impossible to find (due to them being imported from enemy countries and other problems). Black currants are a very rich source of vitamin C (as much as 4 times higher than oranges), and they're suitable for growing in the UK climate.
The British government encouraged black currant cultivation, increasing the English crop very significantly. Almost the whole production of black currants was then used for cordial and black currant syrup, which was given to English children for free (to prevent diseases such as scurvy and keep the population healthy). This led to the traditional black currant flavours found today in Britain
As for the U.S., black currants were once very popular there as well, but in the 20th century the government banned them in almost all of the states. The reason was that black currant shrubs can host and spread a disease, the "white pine blister rust", which threatened the booming timber industry in the early 1900s.
In 1966, the federal ban on black currants was moved to individual States' jurisdiction. This led to many savvy states lifting the ban, as did the New York State in 2003 thanks to the efforts of the Currant Company and Greg Quinn. Other states lifting the ban were Oregon, Vermont and Connecticut, whereas others, such as New Hampshire, Massachusetts and Maine did not lift it as of 2007
The 20th century ban has greatly lowered the popularity of black currant across the United States, and still today blackcurrants in the USA don't have the popularity they enjoy in Europe and the U.K.
After scientific studies have proven the effectiveness of black currants in reducing swelling and inflammation, thanks to powerful nutrients such as anthocyanins, they are now increasing their popularity as a substitute for ibuprofen / aspirin, and as a healthy fruit juice