Actually, if we were to analyze the scenes to see which character is the most dominant speaker and which character most of the scenes are about, we would actually find that the play is mostly divided evenly between Romeo and Juliet, with the exception that Romeo is dominant in one more scene than Juliet.
While the play opens with the Capulets' servants, by the middle of the first scene, Romeo becomes the most important character that we meet. By the middle of the first scene, we have learned about Romeo's important love-lorn state and seen him as a very emotional person in general, which sets the groundwork for plot, theme, and character development.
Juliet is conversed about in the second scene, just like she is in Act 3, Scene 4, but we do not meet her until the third scene of the first act. Since we meet Romeo first and Juliet second, it is safe to say that the play primarily focuses on Romeo.
It is also noteworthy that anytime Romeo and Juliet appear in a scene together, Romeo is the first speaker. Romeo is the first speaker at the Capulet's ball when he asks a servant who Juliet is and then gives a longer soliloquy (I.v.42-55). Juliet does not give her first line until she responds to Romeo at line 102. Again, Romeo is the first speaker in the famous balcony scene, and delivers a long soliloquy beginning at the first line.
However, we do see a switch in character focus in the middle two acts. While Romeo is the primary focus in the first two acts, Juliet is the dominant character in Acts 3 and 4, beginning after Romeo slays Tybalt. In the second scene of Act 3, Juliet delivers a long soliloquy describing her mood as she waits for Romeo and then has a lengthy exchange with her nurse when she learns of Tybalt's death. Aside from Romeo's scene killing Tybalt and crying to Friar Laurence, in the rest of the scenes in Act 3, Juliet is either the main speaker or is spoken about, such as her father speaking again with Paris about marriage. It is also noteworthy that in the scene that takes place the morning after their wedding night, Juliet is the first speaker, asking Romeo, "Wilt thou be gone? It is not yet near day" (III.v.1). Juliet and her family continue to be the center focus. One reason for the switch in focus from Romeo to Juliet is that Juliet experiences character development in both of these Acts. Juliet does a little bit of growing up, learning how to trust Romeo, even in the face of what appears to be deception through Tybalt's murder.
But again in the final act, the focus switches back to Romeo. In the final death scene, Romeo has the longest parting soliloquy, consisting of about 46 lines, while Juliet only has about 11. Hence, through structural analysis, we can say that Romeo truly is the primary focus of the play.