Sati was also a form of the Devi just like Parvati. Why is it that Sati attained Self-realization while Parvati became the consort of Lord Shiva?
Devi (Mother Divine) does not have just one form, She has many. That is why She is known as Leela. She comes in different forms to teach different things, that is her Leela. Just understand this much.