Not uncommon in opera ... The composer choose what kind of voice he wants for the part.

Because Bellini wrote the role of Romeo for mezzo-soprano

Also if look a history and theater, when Shakespeare wrote all his plays, all roles were played by men because women weren't allowed on stage. Convention in opera was to have sopranos or mezzos sing roles of young boys like Cherubino and Octavian who are about 15 in the opera as are Romeo and Juliet. Warriors were usually sung by contraltos because they were older men and a little more dramatic even though in Rossini they had to have very agile voices. Look at the history of castratos and why they were prominent in 17th 18th and part of the 19th century. One even lived into the early 20th century and recorded sacred music. His name was Alessandro Moreschi