On the second night of Pesach, we began counting Sefirat HaOmer, whose source is found in this week’s Parashah, “USefartem Lachem MiMacharat…Sheva Shabbatot Temimot Tiheyena,” “And you shall count from after the holiday…7 complete weeks.” (Vayikra 23:15) Many wonder why the Berachah of SheHechiyanu is not recited on this Mitzvah. One answer, given by Radvaz, is that because Sefirat HaOmer’s purpose is only to lead to the Mitzvah of the offering of the Shetei HaLechem in the Beit HaMikdash on Shavuot. As such, until Sefirat HaOmer is completed, its goal is not attained. Therefore, the SheHechiyanu that we recite along with Kiddush on Shavuot night applies both to the Kiddush and Sefirat HaOmer, as it is finally completed and the Mitzvah of the Shetei HaLechem can be performed.
Rashba offers a second explanation, and states that the Berachah of SheHechiyanu can be said only on a Mitzvah that contains benefit or happiness, like the Mitzvah of Shofar, which connects Bnei Yisrael with Hashem. Because Sefirat HaOmer does not have an aspect of happiness, as the Beit HaMikdash is destroyed, SheHechiyanu is not said. Sefirat HaOmer during the absence of the Beit HaMikdash is only a rabbinic obligation according to most Rishonim (early authorities) since Sefirat HaOmer is triggered by the offering of the Korban HaOmer on the second day of Pesach. The absence of the Korban HaOmer saddens us, thus making it inappropriate to recite SheHechiyanu on Sefirat HaOmer.
Perhaps these two explanations can be reconciled. Nowadays, when the Beit HaMikdash is not present, the SheHechiyanu on Shavuot night may not cover the Mitzvah of Sefirat HaOmer because its purpose does not exist: the Shetei HaLechem are not brought. The absence of the Mitzvah of bringing the Shetei HaLechem causes a lack of happiness in the Mitzvah of Sefirat HaOmer leading up to it. As a result, SheHechiyanu is not recited for Sefirat HaOmer because both its purpose and happiness are lacking.
-adapted from Menachem Beker’s Parperaot LaTorah.