English has become the lingua franca in our world today because it was the native language many of the countries that were most economically and politically important during the last half of the 20th century. During that time, globalization was really increasing and it made sense that the dominant countries’ language would become the lingua franca.
After World War II, the most dominant country in the world was the United States. This was true economically, militarily, and politically. Other English-speaking countries were also rather wealthy and important to the world economy. These included such countries as Canada, the UK, and Australia. Hong Kong, then a British colony, was an important economic power in East Asia.
In addition to this, English was already widely spoken in a number of other countries that were not in the top tier of world economies but had been colonized by English-speaking countries. These included fairly large economies like South Africa and India as well as smaller ones like Nigeria and the Philippines.
Since many of the dominant countries of the world spoke English, and since many of the smaller economies around the world had many English speakers due to colonization, English became a lingua franca.