why was act 2 scene 1 called the balcony scene

I believe the term "balcony scene" originated somewhere between the players and the audience. "Wait until you see the balcony scene..... you'll love it." The fact that Juliet is standing on her balcony oblivious to Rome's presence, only to have one of the most romantic conversations ever makes it the "balcony scene". Whoever named it..... it was an apt description, and it stuck.

This is mostly due to our interpretation of the play. Technically speaking, the original play has a note that says "Juliet appears in a window above." We always use this scene as being on a balcony or with large open windows because it is clear that Romeo can hear Juliet and Juliet can hear Romeo. This would not be possible if it were just a window. Also, when Juliet is speaking to Romeo, she says, "Thou know’st the mask of night is on my face." This means that the night is so dark that Romeo cannot even see her face. This implies that she is outdoors, because it is specifically said as the "mask of night." And, if it is too dark to see her face, then she would not be indoors because the light would shine enough for this. The combination of "appears in a window above", the fact that Romeo and Juliet hear each other, and the idea that the night is too dark to see Juliet implies a balcony. I hope that this helps!