What Juliet has that Rosaline does not?

Romeo's choice is actually made by Rosaline herself. Romeo has a hopeless crush on Rosaline, but first and foremost she has no speaking part in the play; she is an enigma. Rosaline is described as beautiful , but her vow of chastity (she's sworn off men forever) leaves her an unlikely choice. Of her, Romeo says, "O, she is rich in beauty, only poor, That when she dies with beauty dies her store. (1.1.12)

When Juliet comes no the scene she really has no other competition.