Because the entire Kuru clan had to be destroyed. Draupadi was not only born to be an empress or a samragyi she was born with a purpose, she was incarnation of Kali and she was born to destroy the kuru clan and establish Dharma and she knew it.
She never blames Krishna, she never says, โoh Krishna you are my sakha and you made me go through all thisโ
She understands that eventually the great unfolding will be done. She has the last laugh in the end. But without this event it would have been impossible to tilt the balance definitively on the side of the Pandavas.
Affectionately,
Alok Da
