Q. Why don’t we conclude the year’s Torah readings before Rosh HaShanah?
A. When the annual cycle of Torah readings became universal (until then there had been a competing custom to take three years), it must have been decided to complete the annual cycle during Tishri.
However, the special occasions in this month require so many special readings that it would disturb the sequence of Torah portions, so it was decided to link the end of the cycle with the end of the festivals.
There was also an argument that the completion of the cycle should have a festival of its own, which in the Diaspora is the second day of Sh’mini Atzeret and in Israel is combined with Sh’mini Atzeret, which thus offers a kaleidoscope in one day, the sadness of Yizkor and the celebration of the Torah.