The Gemara in Pesachim 115a says that there was a machlokes regarding how one was supposed to eat matzah and marror in the times of the Beis HaMikdash. Hillel said that during those times, when there was a korban Pesach, matzah and marror should be eaten together. His peers argued that they must be eaten separately. The Gemara concludes that since the halacha was not paskened we eat matzah separately, then marror separately, and then both together to accommodate both opinions.
The Rambam (Hilchos Chametz U’matzah 8:8) says that one should korech matzah and marror together, dip it into charosses and eat it without reciting the berachah, “zecher l’mikdash.” The Rambam’s words imply that the reason that we do not recite a berachah on the takanah of korech is because it was instituted zecher l’mikdash (as a remembrance of the Beis HaMikdash). Any takanah that was instituted zecher l’mikdash does not require a berachah.
However the Achronim ask the following question: Originally the mitzvah of lulav was to take the lulav only on the first day, with the exception of taking it in the Beis HaMikdash – where it was taken for seven days. The Mishnah in Sukkah 41a says that after the destruction of the Beis HaMikdash, Reb Yochanan ben Zakai instituted the policy that the lulav should be taken for seven days everywhere, even outside the Beis HaMikdash, as a remembrance of the Mikdash. Why is it that regarding the institution of taking a lulav for all seven days of Sukkos we recite a berachah, and regarding the institution of korech we do not? The rule that we do not recite a berachah on an institution that was initiated zecher l’mikdash should exempt a berachah on the takanah to take the lulav for the remaining six days of Sukkos.
I would like to suggest that the answer lies in the words of the Rambam. The Rambam (Hilchos Lulav 7:15) says that after the destruction of the Beis HaMikdash, it was decided that the lulav should be taken on each of the seven days of Sukkos – zecher l’mikdash. One should recite the berachah of “al netilas lulav” over it every day because it is a mitzvah midivrei sofrim (from the Rabbanan). The Rambam seems to be alluding to this question when he answers that this takanah is a mitzvah. I think this can be better understood based on the following explanation:
The Gemara in Sukkah 44a brings a dispute as to whether a certain rabbinic institution was a takanah or a minhag. The Gemara says that if it is a takanah, a berachah is recited; if it is a minhag, a berachah is not recited. Rashi explains that as the Gemara in Shabbos 23a and Sukkah 46a both say, in order to recite a berachah on a rabbinic decree and say “vetzivanu” (Hashem commanded us) it must be decreed that if one disobeys this ruling he will have transgressed the Torah’s prohibition of lo sasor. Rashi explains that only if one disobeys a takanah will he transgress the Torah’s prohibition; therefore one can make a berachah on it. But if one does not follow a minhag of the Rabbanan, he does not transgress the Torah’s prohibition of lo sasor. As a result, a berachah is not recited.
The Rambam, in Hilchos Mamrim 1:2, says that if one does not adhere to even a minhag he transgresses the Torah’s prohibition of lo sasor. According to this opinion this question remains: Why don’t we recite a berachah on a minhag of the Rabbanan?
The Brisker Rav, in his sefer on the Rambam, explains that there are two requirements that must be met in order to recite a berachah on a rabbinic decree. One is that it was a commandment; this is met when one will transgress a Torah prohibition by disobeying the decree. The second is that its essence must be a mitzvah. Even though one who does not follow a minhag that the Rabbanan instituted will transgress the Torah’s prohibition, it was nevertheless not instituted to be a mitzvah by nature; rather it is a minhag. We do not recite a berachah over minhagim. We only recite a berachah on mitzvos.
By applying this rule we can differentiate between the takanah of taking a lulav on all seven days of Sukkos and that of korech. By the takanah of taking the lulav on all seven days, the actual decree was an institution of a mitzvah in its essence. The reason why this mitzvah was established was zecher l’mikdash. The takanah to eat korech with matzah and marror together was not instituted as a mitzvah, but rather its essence is to commemorate and remember the Beis HaMikdash. The Rambam writes that the takanah of taking the lulav on all seven days of Sukkos is a mitzvah, and therefore we recite a berachah. Regarding the takanah of korech, the Rambam does not say that it is a mitzvah because it was not established as a mitzvah – but rather as a zecher l’mikdash. Thus we do not recite a berachah on korech since its essence is not a mitzvah.