Why would chicken have significant regional differences in flavor?

US poultry farms tend to use a TON of antibiotics... The antibiotics are supposed to stop diseases that would otherwise be rampant in chickens that are housed in close living quarters. (I might add that the USDA swears that it won't approve chickens treated with hormones, but I've seen enough conflicting complaints that I think it's still worth mentioning)

To your question, it's not (IMO) that all US chickens are less full-flavored, it's that your typical normal chicken in the meat case is quite likely been raised on the aforementioned hormones and antibiotics... I've had plenty of free-range (raised in the US) and they are noticably more flavorful than the average...

If you are up for some fun watching, Food Inc repeats a lot of what I've commonly heard about several various US food industries (including chicken): http://www.imdb.com/title/tt1286537/

If you're looking for a true compare and contrast, I'd suggest you buy an "average" chicken and then buy a free-range chicken... You should recognize the free-range version as the same as his international bretheren.

EDIT

As a fair shot for the opposing side, here's a good article from NC State saying why hormones are illegal and pointless to use... That being said, Food Inc showed some pretty mutated birds that were bred for nothing but breast meat: http://www.ces.ncsu.edu/depts/poulsci/newsletter/newsletter_nov04.pdf

Antibiotics might be a reason, but it's not the only cause. Other significant reasons are:

• exercise
• feed
• post-processing

Most of the chicken available in grocery stores in the U.S. is factory raised ... they're bulked up as quickly as possible without threat of predators. They're fed corn, rather than their acting as pest control on farms, where they'd be eating insects, moving about (in their search for insects and other things to eat), and possibly running away from predators (getting more exercise). That's not to say that free roaming chickens wouldn't be fed corn or other processed feed (or even antiobiotics), but that they'd still have an opportunity for other food. The lack of predators means that chickens never have to fly, so they don't need fast-twitch breast muscle, and they can grow to a size where they'd never be able to fly, even for short flight to excape predators.

Also not common in the U.S. are old chickens for stewing ... we have large chickens, but not necessarily the old ones, such as formerly egg-laying chickens that are no longer producing eggs. I have no idea what's done with those ... they're not in grocery stores, so I assume that they're used in some other way (dog food?).

One other possibility is the gender of the chicken. I don't believe that males are raised for meat (or anything else, really, other than in token amounts to sustain the species) in the U.S.. I assume that a higher percentage of roosters would be produced in other countries, but I have no idea if they might be culled early (if there are agression issues, etc, that would make them difficult to raise), like male cows are.

And as for the post-processing comment ... much of the U.S. chicken is sold cut up, possibly with a brine solution injected, rather than being sold whole. This doesn't seem like a big deal, other than the possibility of the brining, but it also means that chickens have been selectively bred for breast meat, rather than whole carcass weight. It also means that most of what we're eating is white meat, rather than a mix of white & dark meat.

Every chicken has differences; breed, food, lifestyle. This also applies to all food (animals and vegetables). These differences make for major changes in taste, texture, food value, and best cooking practises

There are often huge genetic difference in animals and vegetables around the world with the same name. In some case they are entirely different species e.g. pacific island Rail is "chicken" in some outposts (should be Phasianidae family, not Rallidae family)