Charlie: Because there was not such a Chashash for Kalus Rosh in the Azarah during the rest of the year. At first, the Gedolei HaDor in Bayis Rishon and Sheini didn’t think there was a Chashash either. At some point during Bayis Sheini, they realized it was a problem. That everyone agrees to (Bimchilas Kevodo, I think DY misspoke when he said that those who hold it was D’Oraisa said it was there in Mayis Rishon).
Now comes R’ Moshe’s Chiddush. Yes, it is a Chiddush, but it is not beyond the pale of logic as ROB seems to contend. R’ Moshe asks a simple question. He says there is an Issur MiDivrei Kaballah of changing anything in the Beis Hamikdash. And we know that Ein Koach B’yad Chachamim La’akor Davar Min HaTorah B’kum V’asei. Building a balcony, even a temporary one, is being Over on this B’kum V’asei. Hence, it must be that there is a Din Min Hatorah saying to build a separation whenever there is a Chashash of Kalus Rosh. (I will try and find the Mekoros again because I lost the notebook I wrote this down in, but I think I remember once showing that R’ Moshe’s Chiddush could actually be shown as a Machlokes Rishonim; it had something to do with the Ra’avad on this issue. B”N when I get time I will look at the Rishonim again and try to piece together what I once had.)
And ROB, I have to speak out against this. Your nice statement to me rings absolutely false because you are showing more respect for me here than R’ Moshe. Everyone, especially someone knowledgeable, is entitled to disagree with even the Gedolei Haposkim if they feel that they have strong Ra’ayos (R’ Moshe says so himself; maybe they can’t disagree L’ma’aseh, but certainly in learning). But take great care in how you do it. A certain modicum of respect must be maintained, even if no disrespect is intended in your words.