Megillah 31a says the following regarding the Simchas Torah reading:
למחר קורין וזאת הברכה ומפטירין (מלכים א ח, כב) ויעמד שלמה
On the next day [after Shmini Atzeres] we read Vezos HaBeracha, and we read as the Haftarah from “And Shlomo stood up” (Melachim 1:8:22).
Every shul I’ve ever been to, of any nusach, reads the beginning of Sefer Yehoshua as the Simchas Torah Haftarah, a different Haftarah than the one prescribed by the Gemara. Why do we change it?
This question is asked by Tosfos, but they leave it unanswered.
למחר קרינן וזאת הברכה ומפטירין ויעמוד שלמה - ויש מקומות שנהגו להפטיר בויהי אחרי מות משה ושיבוש הוא שהרי הש"ס אין אומר כן ויש אומרים שרב האי גאון תקן לומר ויהי אחרי מות משה אבל אינן יודעין הסברא אמאי שנה סדר הש"ס:
Some places have the Minhag to read as the Haftarah from “And it was after Moshe’s death,” and it’s a mistake, for the Gemara doesn’t say this. Some say that Rav Hai Gaon established that we say “And it was after Moshe’s death,” but they don’t know the reason why he changed from the order of the Gemara.
So what is the answer to Tosfos’ question? Why did Rav Hai Gaon institute this Haftarah instead of the one prescribed by the Gemara?
To clarify: I am not asking why we read from Yehoshua; I am asking why we don’t read from Melachim Aleph.