It's well known that the account of Eliezer, servant of Avraham, is repeated in the Torah, which is not very common.
Regarding this, Rashi brings the Midrash explaining the reason:
" 42So I came today to the fountain, and I said, 'O Lord, God of my master Abraham, if You desire to prosper my way upon which I am going מבוָֽאָבֹ֥א הַיּ֖וֹם אֶל־הָעָ֑יִן וָֽאֹמַ֗ר יְהֹוָה֙ אֱלֹהֵי֙ אֲדֹנִ֣י אַבְרָהָ֔ם אִם־יֶשְׁךָ־נָּא֙ מַצְלִ֣יחַ דַּרְכִּ֔י אֲשֶׁ֥ר אָֽנֹכִ֖י הֹלֵ֥ךְ עָלֶֽיהָ: So I came today: Today I left, and today I arrived. From here we learn that the earth shrank for him [i.e., his journey was miraculously shortened]. Rabbi Acha said: The ordinary conversation of the servants of the Patriarchs is more beloved before the Omnipresent than the Torah of their sons, for the section dealing with Eliezer is repeated in the Torah, whereas many fundamentals of the Torah were given only through allusions. — [Gen. Rabbah 60:8]
ואבא היום: היום יצאתי והיום באתי, מכאן שקפצה לו הארץ. אמר רבי אחא יפה שיחתן של עבדי אבות לפני המקום מתורתן של בנים, שהרי פרשה של אליעזר כפולה בתורה, והרבה גופי תורה לא נתנו אלא ברמיזה:"
The question now is:
On the surface, it seems like the simple reason why it's repeated is to show how important the Avos are, such that even idle conversation of their servants is repeated, while many laws of the Torah aren't.
If so, why doesn't the Torah (also?) repeat at least any of the passages that deal with the Avos themselves?
Why make that which is secondary main, and that which is main, secondary?
A similar, related question, which may be the key to solving the above is, why did Rashi wait all the way into this verse, when Eliezer mentions how he miraculously arrived the same day etc. to explain the reason for why the "conversation of the servants of the patriarchs" is repeated, when the beginning of Eliezer's conversation, which is a repeat of what was said earlier, started earlier, at verse 37?