[Southern Indiana] Why is this so difficult?

I filled in this area of my yard two years ago, and I have been trying in vain to get grass established ever since. The first year I just tilled, seeded and fertilized the fill dirt directly in mid-fall. The grass germinated and grew through the following spring, but then slowly died off over this summer. This fall I tried again. This time I put down topsoil first, used the fancy coated seed, did my seeding earlier in the fall, and put in a temporary irrigation system to get it through the dry fall season. I’ve been regularly mowing and overseeding any remaining bare patches but it just doesn’t seem to be coming in very strong. I’m worried I’ll lose it this winter or next summer. I spread the infill by hand so there are no large rocks underneath, and as you can see I’ve fenced it off to keep the dogs from tearing it up. Why is this so freaking hard?