Seeing as all the Nesi'im brought identical Korbanos, why does the Torah repeat the Korbanos of each Nasi? Why does it not present the details of Nachshon's Korban, and continue 've'Chein Hikriv ... ' for each subsequent Nasi on his day?
Ramban (in Pasuk 2) #1 and Moshav Zekeinim (in Pasuk 84): Hakadosh-Baruch-Hu confers honor upon those who fear Him. Consequently, bearing in mind that all the Nesi'im thought to bring their Korban and to inaugurate the Mizbe'ach simultaneously, and that it was impossible for each Nasi to be the first to bring, Hashem honored all of them by repeating all the details by each one.
Ramban (in Pasuk 2) #2 and Moshav Zekeinim (on Pasuk 84) - both citing a Midrash: Because each Nasi had a different reason in mind - one that was relevant to his tribe. 1
Ramban (in Pasuk 2): #3 Because, based on a tradition that Ya'akov handed down to each tribe what would happen to it until the time of Mashi'ach, each Nasi brought Korbanos that pertained to that tradition.
Seforno: To teach us that each Nasi had in mind to atone for the specific sins of his tribe, and that therefore, each Nasi's Korban was accepted individually with goodwill by Hashem. 2
Oznayim la'Torah: To demonstrate the deep humility and the unity of the Nesi'im who did not try to outdo each other in order to show-off their wealth in front of the people, as is the way of the aristocracy in similar circumstances.
Ramban: Consequently, Nachshon's Korbanos were connected to Malchus, (as we explained - from Pasuk 13 - 18), Yisachar's to Torah and Zevulun's, to Parnasah (See Ramban, DH 'u'Nesanel' and 'u'Zevulun').
Seforno: And each Nasi stood by his Korban - like the Ma'amados -where representatives of the community stood by the Korbanos Tzibur whilst they were being sacrificed on the Mizbe'ach.
Why does the Torah write "Hikriv Korbano" by Nesanel ben Tzu'ar and not be any of the other Nesi'im?
Rashi (in Pasuk 19): Because the tribe of Reuven claimed that, if Yehudah preceded them, they ought to come next, since Reuven was the firstborn. So Moshe informed them that he was merely following Divine instructions - to go after the order of the Machanos, in which case, Yisachar had to follow Yehudah. Consequently, in the following Pasuk, the word, which is missing a 'Yud' can be read 'Hakreiv' ('Bring!') - as if Hashem was instructing Nesanel ben Tzu'ar directly to bring next.
Da'as Zekeinim: According to order of birth, Yisachar should not have offered after Yehudah; they merited to do so however, due to their superior Chochmah. 1
Hadar Zekeinim: According to the order of birth, Nesanel should not have offered after Yehudah, only they pushed themselves forward however, in order to do the Mitzvah.
Sifri: Since Nesanel merited a high level of understanding, 2 the Torah reckons as if he had brought first.
As the Pasuk writes in Divrei ha'Yamim 1 12:33 "u?mi?Benei Yisachar Yod'ei Binah le'Itim".
See answer #2.
