2) The Shu”t Maharil HaChadoshos (21) and Kol-Bo (128) write: The reason we don’t perform birchas kohanim every day in Chutz La’aretz is because it would cause people to miss out on work. Like we don’t add onto the seven aliyas on Shabbos, even though sometimes there are more than seven aliyas needed, as we don’t won’t to take up people’s time. It may also be because of bitul Torah, and tircha detzibbur [bothering the congregation]. Similarly, on Chol HaMo’ed we don’t add on to the four aliyas, as we don’t want to take up people’s time. Therefore, perhaps the same thing is with birchas kohanim, even if the kohanim do it quickly, it still takes time, therefore, they never established birchas kohanim every day.
3) The Shu”t Maharil HaChadoshos (21) offers a second reason: It use to be common for non-Jews to be in shul, and birchas kohanim is only for Jews, therefore, they said it’s best not to say it (see also Mateh Moshe 199, cited in Elyah Rabbah 128).
4) The Shu”t Chasam Sofer (Orach Chaim 23) explains: The mitzvah of birchas kohanim is mentioned in the pasukim after avodah, like we learn in Megillah (18a): וירד מעשות החטאת ויברך את העם - “Aharon finished the avodah and then he blessed the people”. Tefillah is in place of the avodah, ואין ספק כשאין עבודה רצויה והיא פגול ח"ו גם ברכת כהנים לא תחול על המתברכים אז – “There is no doubt, that if the avodah is not done in a pleasing manner, and it is pigul chas vesholom, then the birchas kohanim is also not very good, and won’t work for those being blessed. והיות בעו"ה כל ימות החול טרודים על המחיה והכלכלה ורוב התפלות בלי כוונה וטרדה מרובה ותפלה בלא כוונה כקרבן שא ינו רצוי על כן מברכים ביו"ט שהעולם פנויים ומכוונים בתפלתם - “Unfortunately, nowadays, we spend our days of the week busy with parnosah, and most of our tefillos are without concentration as we have lots of things on our minds. Tefillah without concentration is like a korbon which is not very pleasing to Hashem, therefore, we only perform birchas kohanim on Yom Tov, when people are not busy with work and they have better concentration in their davening.”
However, the Shu”t Binyan Shlomah (10) asks: Birchas kohanim is de’O’raisa, whereas tefillah is only derabonon. Even according to the Rambam that there is a chiyuv de’O’raisa to daven every day, the Rambam himself (Hilchos Tefillah 1:2) writes that until the days of Ezra, there was no fixed text, and they wouldn’t daven with a minyan. If so, it can’t be that the mitzvah of birchas kohanim is connected to tefillah. The Biur Halachah (128) also writes, that it doesn’t make sense for birchas kohanim to be related to tefillah, as tefillah is derabonon.
5) The Shu”t Beis Ephraim (Orach Chaim 6) explains: The lineage (yichus) of kohanim nowadays is not well traced and the only reason we treat them as kohanim is because they have a chazakah. Therefore, they fixed that we don’t duchen every day as it may be a blessing in vain, as well as a problem of a non-kohen duchening. Really, they should have nullified the mitzvah altogether, however, כדי שלא תשתכח תורת הכהונה ולא יבואו לזלזל באיסורי כהונה – in order that the laws of kohanim don’t become forgotten, and people don’t treat kehunah disgracefully, they enacted that at least on Yom Tov kohanim duchen.
6) The Shu”t Rema MiPano (95) explains: We darshen, וישא אהרן את ידיו וירד מעשות החטאת – “Aharon raised his hands (i.e. duchened) and came down from offering the chatos”, to learn, that we only perform the mitzvah of birchas kohanim on Yom Tov during Mussaf, when we bring a korbon chatos.
7) The Rema (see Darchei Moshe 128) explains: People are busy with work, and they are not happy, even on Shabbos people are worried about their work. On Yom Tov, however, there is a mitzvah of simchas Yom Tov, and people are less worried, therefore, in Chutz La’aretz we only perform birchas kohanim on Yom Tov.
8) כיון שמעלין מדוכן ליוחסין משום דאי לאו כהן הוא כולי האי לא מחציף נפשיה – “If we see a person duchening we assume he is a kohen, as if he wasn’t a kohen he wouldn’t have the brazenness to go up and duchen.” However, this is only on Yom Tov when there are lots of people in shul, and one is scared to be brazen in front of so many people. (See Hafla’ah to Even HaEzer, Nesivos L’Shabbos 3a)
9) The Shu”t Eretz Tzvi (1:3) explains the minhag based on the Gemara in Eruvin (65a). The Gemara presents an argument which would exempt anyone from being punished for not davening properly. The Gemara says, from the time of the Churban, Klal Yisroel were subject to such tragedy, they became drunk even without wine, and one who is drunk is exempt from tefillah. It’s forbidden for a kohen to duchen whilst drunk. Since in golus we are drunk, kohanim don’t duchen. Yom Tov, however, is a time of simcha and people aren’t drunk from the galus, therefore, we perform birchas kohanim on Yom Tov.
10) R’ Shamai Gross shlita, offers an additional reason: The Zohar cited in the Magen Avraham (Hilchos Nesiyas Kapayim) says, if a kohen has an enemy he is not allowed to duchen. In Chutz La’aretz hatred is very frequent (see Nedorim 22), therefore, they established that birchas kohanim is not recited every day. In Eretz Yisroel, however, there is less hatred, therefore, we duchen every day.
The Netziv in Shu”t Meishiv Dovar (2:104) writes: I remember hearing from the Gava’ad of Volozhin that once the Gaon wanted to start performing birchas kohanim every day in his beis hamedrash, and Shomayim stopped him, and he was taken to prison. After that, another rov tried to perform birchas kohanim every day, and the night before, a fire broke out and half the city and the shul was burnt. After these two incidents: וראו שיש בזה איזה דבר סוד בסתרי השפעת הברכה היורד ע"י ברכת כהנים בחו"ל ואין אתנו יודע עד מה - “They realized that there is some deep reason behind not performing birchas kohanim in Chutz La’aretz, and we don’t know exactly what”.
The Aruch HaShulchan (Orach Chaim, 128:4) writes: אין שום טעם נכון למנהגינו לבטל מצות עשה דברכת כהנים כל השנה כולה – “There is no good reason for our custom not to perform birchas kohanim all year round”. However, he then brings the two stories we mentioned above, and says that there is like a Bas Kol [Heavenly voice] telling us not to.
