In Bereishis 3:8, Rashi makes the following comment about his method of interpretation:
וישמעו. יֵשׁ מִדְרְשֵׁי אַגָּדָה רַבִּים וּכְבָר סִדְּרוּם רַבּוֹתֵינוּ עַל מְכוֹנָם בִּבְ"רַ וּבִשְׁאָר מִדְרָשׁוֹת; וַאֲנִי לֹא בָאתִי אֶלָּא לִפְשׁוּטוֹ שֶׁל מִקְרָא וּלְאַגָּדָה הַמְיַשֶּׁבֶת דִּבְרֵי הַמִּקְרָא דָבָר דָּבוּר עַל אֳפַנָּיו:
And they heard: There are many Aggadic midrashim, and our Sages already arranged them in their proper order in Genesis Rabbah and in other midrashim, but I have come only [to teach] the simple meaning of the Scripture and such Aggadah that clarifies the words of the verses, each word in its proper way.
Why does he specifically comment on this passuk? Why not say this at the beginning of his commentary? What prompts Rashi to say this here?