Kuina managed to release Wado's Bankai that killed her in the process because her haki was not enough.
On a serious note, I think she might have committed suicide. Perhaps she realized she could never become the World’s Strongest Swordsman and feared that staying alive would only hold Zoro back from achieving his dream. She may have known that one day Zoro would surpass and defeat her, but more than that, she feared that if Zoro chose to stay by her side, the fire driving him to become the World’s Strongest Swordsman would fade — that their friendship would overshadow his ambition, making the dream feel less important than their bond. From her perspective, she probably felt the need to make a sacrifice to ensure Zoro stayed fully committed to his path — much like Shanks did in his own way. Given the strong Japanese themes around honor and duty, it’s possible she saw suicide, akin to seppuku, as the ultimate act of sacrifice to preserve Zoro’s dream.