It started when the Palace of Westminister was destroyed by a fire in 1834. When they started to rebuild it in 1844, they wanted to add a tower and a clock. Their first attempt failed, resulting in a crack in the Bell.
Big Ben was connected to the Palace of Westminster, they wanted something to compare and make the palace better. So, they formed the Great Bell, which makes it a great tourist site for people. Over years, people called it Big Ben.