Why do networx use a low-rated, already-cancelled show for a filler, especially during the regular season, knowing full well they're gonna take a viewership hit in that time slot, & perhaps in the next one as well? (Not counting Burnoff Theater in the summer)

Is it because they have nothing else to offer, which might do better? I mean, shoot, it's not as if someone stepped in the cable, interrupting the scheduled show; they've had timeta think it over. Is it because contractural obligations with the sponsors, or with the producers/cast of that show?

I am hoping some of you here who are closer to the business end of net TV can answer.