Phasing in healthier stuff like salads, wraps, whatever, is gambling that you'll pull in a new customer base.
It seems their reputation was already so low that that target base didn't believe in the new offerings, didn't believe a company that thought deep fried cardboard shit is food could also dabble in genuine food. Just as if Yugo came out with a new car model called "OK we got it right this time", car buyers wouldn't flock to buy it.
That's where they went wrong -- establishing the low bar in the first place. Had they, maybe, rebranded some stores with a new name they might have been able to pull it off. It's all about perceptions.