No one wants chickens to be able to fly well. People want them to be fat, easy to keep in one area, and tasty. Therefore, they breed the fat, tasty, manageable ones. Chickens may have been able to fly a couple thousand years ago but they can't now because humans have selectively bred them.
It might be better for an individual chicken's health for it to be able to fly, and therefore escape being slaughtered, but evolution doesn't work like that. The chickens carrying genes that people like are the ones that have been allowed to survive long enough to breed, so useful chickens are all that's left. Selective breeding is just an extension of natural selection - those that humans deem fittest to eat and sell are the ones that have offspring.
Feral chickens might get back their ability to fly if there was selective pressure to do so. If chickens with larger, more usable wings were more likely to breed, then those individuals would have more offspring than others and might be able to regain flight in a matter of a few centuries. Keep in mind though, that these chickens get eaten too, whether by humans or by other predators. I don't think a slightly larger wing would help them much, chickens would have difficulty getting over that hump to become able to fly again.
Bird/dinosaur transitional fossils lack many characteristics that modern birds have. They might have evolved large feathers for a variety of reasons other than for flight, to get mates, to keep warm, or for whatever reason. Early birds couldn't fly like most modern birds can either, they could only glide, much like flying squirrels do.
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