To my knowledge, when she was introduced on L&O - it was said that she was the interim DA. So in essence, she wasn't necessarily meant to stay.

I like Diane in many things, but I don't think she was well cast in Law and Order. She was just too soft-acting for my taste, for someone who was a DA. The other women on the show were much stronger characters. She just seemed out of place, to me.
The first DA, Steven Hill, was so much better, and so was Fred Thompson, though I despised him in the Sentate; his most ridiculous casting. He was much better as the Congressional Committee Lawyer who elicit5b4ed the existence of the Nixon taping system in the White House, from Butterfield, I believe it was. That, he did in real life.

Yeah I think I read somewhere she didn't like being committed on something for so long, and decided to quit.
Blanchett / Bullock / Exarchopoulos / Thompson / Winslet

Thank you. I was just curious.

I'm tired of my job. But i'd ve a fool to quit it with my bills and rent

I believe it was because she tired of the role and asked to be written out.