To fully answer this question we need to look back at where she came from and how she achieved that title. Beyonce first began singing as a very young child in a Montessori school. She was noticed by her dance instructor when she was humming along to a song and could hit high notes flawlessly.
When she was 8 years old she became part of the girl band which would eventually be named Destiny's Child. The group released several hit songs and became an iconic group of the 2000s.
Beyonce began her solo career in 2003 when she broke away from the group. After so many hits and her continuing success in her solo career, it makes sense that she would earn this title.