Why do some people think that Shakespeare did NOT write Romeo and Juliet?

They question whether someone from his class and with his education could be so brilliant.
They have documented, indisputable proof that "Shakespeare" was just a stage name for Francis Bacon.
Plays back in Shakespeare's time evolved out of an oral tradition. There is no one "author" of any Elizabethan plays.
The Earl of Oxford claimed in his autobiography—discovered years later—that he is the true author of Shakespeare's plays.