Why did Yaakov not accept Reuven's offer? Reuven accepted a severe punishment should he fail to return Binyamin! Later, Yaakov did accept Yehudah's offer (43:11).
Rashi: He dubbed Reuven his foolish Bechor for offering his two sons to be killed - as if they were not his (Yaakov's) sons too!
Ramban (to 42:37) #1: Because Yehudah was the strongest (leader) of the brothers.
Ramban (to 42:37) #2: Because Yaakov did not trust Reuven, who had already sinned against him once. 1
Ramban (to 42:37) #3: Initially, he would not have accepted Yehudah's offer either, and it was only because Yehudah wisely waited until there was no bread in the house before offering to take Binyamin, at which point Yaakov had no choice.
Avos de'R. Nasan: The difference between Reuven's offer and that of Yehudah was that Yehudah's offer, "Anochi E'ervenu ... " was to the point ('Sho'el ke'Inyan') whereas Reuven's was not ('Sho'el she'Lo ke'Inyan').
Ohr ha'Chayim (to 42:37): Reuven did not accept a loss in the world to come; whereas Yehudah did.
Even though he had done Teshuvah! (EC)
Shimon was security for the brothers to return with Binyamin. If they would not, Shimon is Chayav Misah. Why was Yaakov unwilling to risk loss of Binyamin in order to surely save Shimon?
Ohr ha'Chayim #1: This is why he said "his [maternal] brother died" - there is more danger to Binyamin.
Ohr ha'Chayim #2: This is why he said "he is the only one remaining from his mother" - loss of Binyamin is worse than loss of Shimon.
Malbim: Yaakov refused due to his bad Mazal (already two sons are missing). Reuven said that he accepts responsibility, so it does not depend on Yaakov's Mazal. Yaakov answered that there is bad Mazal due to Binyamin's mother - he is the only remnant from her.
Ha'amek Davar: Perhaps I would accept Reuven's offer to go elsewhere, but not to Egypt. 1 Ohr ha'Chayim - His brother died "on a path that you go" [regularly, from Chevron to Shechem] - all the more so there is concern for going to Egypt!
Ha'amek Davar did not explain why Egypt is worse than other places, i.e. due to the viceroy who said that you are spies, or due to the immorality and witchcraft there. (PF)
QUESTIONS ON RASHI
Rashi writes: "He did not accept Reuven's words; he said, 'This one is a foolish Bechor!'" How do we know this was Yaakov's intent?
Gur Aryeh: Yaakov did not respond to Reuven's words in any way; he merely re-iterated his previous position (42:36) that Binyamin would not go with them.
Rashi writes: "He did not accept Reuven's words." Why did Yaakov elaborate with a reason, "... for his brother has died, and he alone remains..."?
Gur Aryeh: Reuven was likely to think that his offer was in the right direction, yet was insufficient to convince Yaakov to send Binyamin. Yaakov repeats the fear he already expressed (in 42:36), to show that Reuven's words were irrelevant, and he dismisses them.
Rashi writes: "He did not accept Reuven's words." Perhaps Yaakov did not accept Reuven's suggestion, only because it would be impossible to fully guard and protect Binyamin against all possible harm along the journey?
Gur Aryeh: Yaakov was afraid because "the accuser prosecutes in times of danger" (Rashi to 42:4); but yet, it would be possible to guard Binyamin properly and alleviate the danger. Yaakov later agreed that Binyamin would travel with Yehudah, who put his Olam ha'Ba on the line to protect him (43:9).
