Why did Lori Alan leave Family Guy?

Answer 1

Nobody here can possible know the answer as to her decision to stop voicing the character, or even if it was indeed her decision.

Since the character had become underutilized, either by Lori being too busy to provide the voice or the makers having no use for the character, in the seasons prior to And Then There Were Fewer it's a safe presumption the makers of Family Guy just decided to kill Diane off.

Answer 2

Lori (the character) didn't leave. After delivering her daughter, she died. Carl stabbed her corpse in the head to keep her from reanimating, and one of the walkers ate her body. Rick killed that walker when he went looking for her and could not find her.