not enough light.
only the new leaves around the outside of the rosette died..so your roots are fine.
you're watering properly, which is the hardest part to get right. congratulations.
it's not getting enough sun.
remember, everywhere inside your house is dense shade. watch where the sun comes through your windows and crosses your floor, and put the plant directly in the spot, close enough where all of the leaves will throw a shadow.. the leaves need light, not the roots. if you watch the shadow you'll know where light is hitting or not. luckily these don't need much.. 3-4 hours a day of indirect sunlight will be fine
or, the plant will continue to die back and you can repot into a little pot and have a 16" peace lilly instead of a 3 footer, then a 8" plant, then a smaller pot...
it's really up to you. often we have to trade off a spectacular plant in a bad spot, or a plant in a spot that's pleasing to the decorator's eye.. it's really up to you. if you leave it without light it will shrink back and loose leaves till it dies. good news is that will take a year or two, since it looks like you're watering correctly . A bouquet of lilies will last 5 days. you have tons of time.
