Why is it only on Leil HaSeder and Kol Nidrei Night the only 2 times we say, ” L’Shana Habah BiYerushalayim”?
One answer can be that on Pesach we find that although we have a night full of mitzvos, we nevertheless are missing THE MAIN mitzva of Yom Tov, namely the Korbon Pesach. Most of the mitzvas of Pesach revolve around the korbon pesach (i think the Gr”a made a cheshbon of 53 mitzvos, I think). Most of mesechta Pesachim deals with the Korbon Pesach.
By Yom Kippur, of course, the main part of Yom Tov is the Avodah of the Kohen Gadol going Lifnai V’lifnim, offering the Ketores in the Kodesh HaKedoshim. That was the highlight, and Main mitzvah of Yom Kippur.
By all other Yomim Tovim the main mitzah still is in effect.
Rosh hashana- we still have shofar (although the Korbon Mussaf is missing, we still have the main mitzva of Shofar)
Sukkos- we still have Suka and 4 minim.
Shavuos- we still have the torah.
So it’s really only these 2 times of the year, Pesach and Yom kippur, which the MAIN AVODAS HAYOM is missing because of the lack of the Bais Hamikdash. So it is specifically on these 2 occasions where we pray, “L’Shana Habah B’Yerushalayim” for the binyan Bais Hamikdash.