Two things about the Pandavas needed explaining even to the epic's first audiences, and it explains both at length.
The first is their parentage. Pandu is under a curse that will kill him if he approaches a wife, so Kunti uses a mantra given to her in girlhood by the sage Durvasa, which summons a god who must give her a son. She uses it for Dharma, Vayu and Indra, and teaches it to Madri, who summons the Ashvins and gets twins. The device lets the poem give five brothers five divine fathers without making them illegitimate, and it is the reason Duryodhana can say in council that they are not Pandu's sons at all.
The second is the marriage. Arjuna wins Draupadi at the archery contest, the brothers bring her home, and Kunti, without looking, tells them to share what they have brought. That her word cannot be unsaid is offered as the immediate cause, but the epic clearly does not think it sufficient, because Drupada objects on the spot that a woman with five husbands is against custom, and the poem then supplies three further justifications: the practice of the elders, a previous birth in which Draupadi asked Shiva five times for a husband and was granted five, and a divine origin for the five brothers themselves.
An epic does not argue this hard about something its audience found ordinary. The four defences stacked on top of each other are the clearest evidence that polyandry was already unusual when the text was fixed.