Although it is not known why she specifically chose to use her middle name as her stage name, it is likely because it has more of a ring to it than Robyn.
Robyn Rihanna Fenty was born on February 20, 1988, in Saint Michael, Barbados. Despite now being famous and living in the U.S, she was actually raised in Barbados.
Rihanna started to become famous after her debut studio album in 2005, Music of the Sun. She followed this album up in 2006 with her second album, A Girl like Me.