There is a 'but' in that:
all animals require nutrition, be it vitamins and minerals, protein, fats and carbohydrates.
Bread is a ball of carbohydrate, pretty much instant energy; the first nutritional requirement to be fulfilled is energy.
Carp will eat most things, or try them.
Forget the word bait for a minute, change it for food. There you have the nutritional requirement, protein, fats, carbohydrates.
If you can provide a food that the carp can eat long term, you are supplying (as an example) every week, then they may actively search it out. In fact, you can prove this with farm fish, throwing pellets in, they are ready and waiting, and will eat them every time. No fishing pressure, but a food.
Now swap it back in as bait.
A fair few years ago at Taverham I baited up a fishmeal bait every day in a particular spot and time. (The joys of running the place).
There were fish that were there almost every day, and feeding as soon as the boilies went in. I could drop boilies in on top of them, and they ripped the bottom up. Another area I baited regularly, the carp would feed heavily in the area, not just my bait, but tearing up the bottom for bloodworm as well.
I could fish this bait in pretty much every swim, and know the carp would take it readily if they found it.
Now we come to why carp may stop taking a particular boilie. You simply aren't feeding enough to get them to eat comfortably. They have become wary. They know the food is good, but if every time they eat it a big predator hooks them, stresses them out and lifts them out of the water, may start to leave it.
Prebaiting again regularly may be enough to get them feeding and taking the bait as food again, or in many anglers cases it is the reason to change baits.
I have tried using baits made from liquidised maggots, casters and worms. The real thing produces more fish than boilies, but small fish can be a nuisance.