Why were all the Upapandavas killed the same day?
Why was no one, including Lord Krishna, able to save them?
Why was only Parikshit saved by Krishna from Uttara's womb?
Was there any story behind their short lifespan?
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The superficial reason as to why the Upapandavas (the five sons of Draupadi) were killed is simply that Krishna and Pandavas weren't at the camp the night Ashwatthama attacked, as described in this chapter of the Sauptika Parva of the Mahabharata:
Through fear of the Parthas, O son of Kuru's race, Ashvatthama could not achieve such a feat then. It was owing to the absence of the Parthas and the intelligent Keshava as also of Satyaki, that Drona's son could accomplish it. Who is there, the lord Indra unexcepted, that is competent to slay them in the presence of these heroes?
But the ultimate cause of their death is described in this excerpt from the Markandeya Purana.
One of the most famous Vedic kings was Harischandra, a solar dynasty king whom I discuss here. Similar to the story of Job in the Bible, the gods once decided to test Harischandra's virtue by depriving him of everything he had. In particular, the sage Vishwamitra asked Harischandra for his entire kingdom, and out of a sense of duty he agreed to give it up. Now five of the Vishwadevas, a group of Vedic gods, saw what was going on and were appalled that Vishwamitra would torment such a righteous king, so they spoke out against him. Vishwamitra was enraged by their words, so he cursed them to be born on earth, but he lessened it by allowing them to return to Devaloka before they got married or had kids. Thus they were born as the Upapandavas:
But then spoke five Vishwadevas in pity, "This Vishwamitra is very wicked; what worlds will he obtain, who has uprooted this best of sacrificers from his throne? By whose funeral ceremony further shall the soma juice expressed at the great sacrifice by purified, by drinking which shall we reach the exhilaration that is preceded by incantations?" ...
Having heard their remark, the sage of the Kaushika race, exceedingly enraged, cursed them -- "Ye shall all assume human form." And propitiated by them, the great Muni added, "Although in human form, ye shall have no offspring. There shall be neither marriage nor wives for you, nor hostility: freed from love and anger ye shall be gods again." Thereupon those gods descended to the mansion of the Kurus with their own portions; they were born from the womb of Drauapdi as the five grandchildren of Pandu. Hence the five heroic Pandaveyas did not take to themselves wives, through the curse of that great Muni.
As I remember from a folk tale, they were reincarnations of the guardian deities cursed by Viswamitra. Below is the full tale:
The sage Viswamitra decided to test if king Harischandra really deserved his reputation as a most truthful king. He managed to trick the king into giving up his Kingdom and everything in it, including his clothes and ornaments and forced the king to leave, along with his Queen and son. the sage actually beat the queen to push them out of the kingdom. The five guardian deities of the kingdom were enraged by this and condemned the Sage. The sage became enraged and cursed them to be born as mortals. Once they begged Viswamitra’s pardon, the sage revised his curse, saying that though they would be re-born as mortals, they would get their moksha before forming any attachments that would reduce their status and sanctity as guardian deities.
And hence, the guardians were born as the sons of Draupadi, who were killed before they were old enough to earn a name or marry and have any progeny of their own. Their souls attained relief from Sage Viswamitra’s curse.