Because otherwise airlines buy different planes. All airplane models have extremely detailed maintenance schemas with alternative procedures described where possible. And minimum equipment lists that describes exactly what must work and what is "okay" to be broken to still fly. And it's on FAA to make sure Delta is following these manuals. So in the end the blame is on Boeing for either bad parts, lasting shorter than required or prescribing insufficient maintenance procedures. Or it's on FAA for not doing ther duty in making sure the procedures are followed. Of course if Delta hasn't followed the procedures, blame is on them too, but only ever in combination with either Boeing or FAA.