Why do we change the Haftarah for Simchas Torah?

Question

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.

Answer 1

R. Asher Ben Yechiel cites the Jerusalem Talmud as the source for reading the beginning of Yehoshua as the haftarah:

Rosh Megillah 4:10

ביו"ט אחרון של חג קורין כל הבכור ומפטירין ויהי ככלות שלמה ולמחר קורא וזאת הברכה ומפטירין ויעמוד שלמה לפני ה' וי"א ויהי אחרי מות משה עבד ה' והכי איתא בירושלמי והכי נהגינן

This is codified by his son R. Yaakov Ben Asher:

Tur O.C. 669

ובירושלמי יש שמפטירין ויהי אחרי מות משה והכי מנהגינו

So to start, the question is somewhat mitigated. It's not like someone made up this haftarah out of nowhere if it's actually the haftarah prescribed by the Jerusalem Talmud.

The slight problem with this is that, as R. Netanel Weil pointed out in his commentary to the Rosh, this passage does not appear to actually exist in the Jerusalem Talmud.

However, in his commentary to the Tosafot you quoted, R. Yaakov Kamentzky tries to answer Tosafot's question by advancing a theory for how our custom developed. He suggests that this is intertwined with the triennial Torah-reading cycle. He argues that when the Torah was only finished every third year, the topic of the completion of the Torah was not as relevant to Shemini Atzeret/Simchat Torah, and therefore they would read the haftarah from Melachim. Once the triennial cycle was abolished, R. Hai Gaon came along and decided that since Simchat Torah is now always the day when we complete the Torah, it makes sense that the haftarah should be the beginning of Yehoshua which demonstrates the connection between the end of the Torah and the beginning of Neviim, and is the continuation of the story after Moshe's death.