This question has a link in one of the comments referring to an article explaining why each day of Pesach has a different Torah reading.
During Succot, I notice that the 2nd day of Yom Tov in Diaspora repeats the reading of the 1st day, whereas in Israel, the reading is different, and is only from the Succot sacrificial offerings mentioned in parshat Pinchas. I assume that in Diaspora, the reason we repeat the 1st day's reading is respect to Yom Tov, as we treat the 2nd day as the first? Technically, what would be wrong with reading the sacrificial portions as they do in Israel. and just repeating the readings of some sections, if needed. (We do this, anyway, on all days of Hol Hamo'ed Succot.)
Another way is to view this in "reverse". On the 2nd day of Pesach, we could repeat the reading of the 1st day in Diaspora. Yet, here, Israel and Diaspora have the same reading, except for the number of aliyot, where Diaspora has 5 and Israel has 4.
Why the difference in the reading rules between Pesach and Succot?