Fast food competition is a real dog-eat-dog world (it's just a saying please don't eat dogs none of these places have dogs to eat don't cancel me please I can't afford an internet scandal I have too much to lose).
The point is, people aren't looking for quality when they get fast food so much as they're searching for convenience and a good deal. If you can get me my burger and fries fast without denting my wallet, I'm there.
Still, certain fast food products tend to sell better than others — just look at Taco Bell. They've kind of got a monopoly on fast food tacos, so much so that we buy them in whatever quality they're presented to us in.