Why does Rashi question only the Darkness

Question

Why does Rashi ask why the plague of darkness was brought upon them, but did not ask the same by any of the other plagues?

Text of Rashi 10:22:

ולמה הביא עליהם חשך, שהיו בישראל באותו הדור רשעים, ולא היו רוצים לצאת, ומתו בשלשת ימי אפלה כדי שלא יראו מצרים במפלתם ויאמרו אף הן לוקין כמונו. ועוד שחפשו ישראל וראו את כליהם, וכשיצאו והיו שואלין מהן והיו אומרים אין בידינו כלום אומר לו אני ראיתיו בביתך ובמקום פלוני הוא

Answer 1

Before answering your question, I will strengthen it. Rashi is channeling midrash, as he so often does. And by other makkos, the Midrash Rabba does give a reason, in the same form. Thus, immediately preceding is arbeh, locusts. And we read there in the midrash:

ויאמר ה' אל משה נטה את ידך ארבה למה הביא עליהן? מפני ששמו את ישראל זורעי חטים ושעורים, לפיכך הביא עליהן ארבה ואכלו כל מה שזרעו להם ישראל.

and yet Rashi does not seem to mention this middah keneged middah aspect. However, the midrash says by choshech as follows:

חשך למה הביא עליהן? יתברך שמו של הקדוש ברוך הוא שאין לפניו משוא פנים והוא חוקר לב ובוחן כליות, לפי שהיו פושעים בישראל שהיה להן פטרונין מן המצריים, והיה להן שם עושר וכבוד ולא היו רוצים לצאת. אמר הקדוש ברוך הוא: אם אביא עליהן מכה בפרהסיא וימותו יאמרו המצריים: כשם שעבר עלינו כך עבר עליהן, לפיכך הביא על המצריים את החשך שלושה ימים, כדי שיהיו קוברין מתיהם ולא יהיו רואין אותן שונאיהם ויהיו משבחין להקדוש ברוך הוא על כך.

I would suggest that while the vast majority of what Rashi says is channeled midrash, he channels the midrash in order to answer peshat-based concerns. Thus, his famous methodological statement of ואני לא באתי אלא לפשוטו של מקרא ולאגדה המיישבת דברי המקרא דבר דבור על אופניו. (Though I interpret that in a different manner elsewhere. And see here.) So, the question might be, what does citing this midrash add, as a peshat concern?

I would answer that this is because of the chamushim comment he will make later, in Beshalach (13:18). The pasuk:

וַיַּסֵּב אֱ־לֹהִים אֶת הָעָם דֶּרֶךְ הַמִּדְבָּר יַם סוּף וַחֲמֻשִׁים עָלוּ בְנֵי יִשְׂרָאֵל מֵאֶרֶץ מִצְרָיִם:

and Rashi writes:

וחמשים: אין חמושים אלא מזויינים. לפי שהסיבן במדבר גרם להם שעלו חמושים, שאלו הסיבן דרך יישוב לא היו מחומשים להם כל מה שצריכין, אלא כאדם שעובר ממקום למקום ובדעתו לקנות שם מה שיצטרך, אבל כשהוא פורש למדבר צריך לזמן לו כל הצורך, ומקרא זה לא נכתב כי אם לשבר את האוזן, שלא תתמה במלחמת עמלק ובמלחמות סיחון ועוג ומדין, מהיכן היו להם כלי זיין שהכום ישראל בחרב. וכן הוא אומר (יהושע א יד) ואתם תעברו חמושים. וכן תרגם אונקלוס מזרזין, כמו (בראשית יד יד) וירק את חניכיו וזריז. דבר אחר חמושים אחד מחמשה יצאו, וארבעה חלקים מתו בשלשת ימי אפילה:

If he is going to be establishing it within the midrashic narrative later on, he might as well cite the midrash in its local place as well, to provide further support and a consistent structure.

In other words, he will say later that only 1/5 left, on chamushim, so it is a good idea to prepare his reader and solidify his commentary by noting the midrash that 4/5th died during the plague of Darkness, in its place.

Answer 2

There are quite a few answers here is one from the Terumas Hadeshen. He says the reason only at Darkness does Rashi specifically say why darkness is that since it lasted 6 days it would seem HAshem broke his promise that "day and night Shall Never Cease"(Berisheis 8:22). It actually was not as the Jews had light and the promise was it would not stop for all of the world but for Parts Like ALska at ceartin points it is not a problem. So to allow the Evil ones to be killed and to answer why not a breaking of promise Rashi Had to Provide answer.

Answer 3

Following the כללי רש"י from the Lubavitcher Rebbe, this is the way it appears to me.

Rashi's initial question is, since he previously established that the Makkos were 7 days each, with 21 days of warnings, the immediate question here is why is this makka only 3 days? He then answers that there were two three day periods of different types [one in this verse, one in the next]. [The seventh day would be the day the Makka started, as the previous Rashi explained that the night being worse was part of the Makka, so here since it started by day (as Hashem only spoke to Moshe by day), and Miktzas Hayom Kekula to count to 7, but for the three day periods you would expect them to be equal, so the three days of each are three full days].

This follows the rule that Rashi expects the student to remember what was previously said.

This then raises the question - why is this Makka divided into two parts, which is different than the other Makkos? This Rashi answers with a different question - what is the purpose of the whole Makka? In the answer he gives two reasons, which correspond to the two periods. The first reason (to bury the dead) requires only that it be dark so that they couldn't see. The second reason (to investigate the possessions in the Egyptians' houses) requires that they be immobilized in their houses.

This follows the rule that Rashi never states the student's question, he just answers it. When he asks a question it is because he expects that the student has a different question, and in order to answer that one, Rashi asks a separate question, the resolution of which answers the question which prompted Rashi's explanation.

From this the answer to your question is that Rashi isn't out to explain the purpose of the Darkness as a stand alone question, rather he is bringing this explanation of its purpose in order to understand why it had two stages.