As an Olympic weightlifter, I can give you the answer to this.
1 - the countries that do well in general have a larger pool of atheletes who train full time. In the US, weightlifting is not, in general is not treated as a "serious sport", a much smaller number of people compete.
2 - the countries that do well in general have better training methods. The typical US weightlifter, either trains once a day (often after a full day at work), and some national level competitors only train 3-4 times a week. The countries that do well in general have in general 1-2 training sessions per day, 5-6 time a week, and some even have 4-5 training sessions a day. Yes really -I know this for a fact - for the countries such as Bulgaria (unfortunately their whole team did not compete this year) each training session is very intense, and might last 20-45 mins. Eg morning session 9am 20 mins shrugs/high pulls. 12pm 30 mins half squat snatches, 3pm 30 mins jerks, 6pm, 45 mins heavy deep squat session.
3 - the countries that do well have really great coaches. They outdo the US in training technique, stretching/flexibility training, lift training, but also in using training related exercises, such as jumps, as well as recovery therapies such as massage/sauna etc.
4. the countries that do well in general have people who can really just eat, sleep, train, and recover. 100% focused on creating a body that will achieve record lifts.
5. the countries that do well in general actually select people who are suitable to be future stars at an early age. Body shape, bone density, speed, flexibility, explosiveness, strength etc, whereas for the US, there is much less of a national selection process.
6. the countries that do well in general have way more female competitors. In the US, its really still pretty rare for a women to be a weightlifter.
OK, and now finally a point on performance enhancement. Unfortunately (depending what view you take on this) the US is now pretty strict on drug testing, which makes it pretty hard for competitors to use substances in a way that gives them a significant benefit without getting caught.
Compare this to countries such as China, Greece, Iran, Poland, Russia, Bulgaria, India, etc, where athletes can take (or are given) what they need. They key substances for a weightlifter are, depending upon age, sex, and stage of development, testosterone, winstrol, and increasingly GH. (Other popular substances, are dbol and also deca, among others).
In the countries that do well in general, an athelete can, leading up to a major contest do a strength phase for half year test/gh, and then spend the next 3 months moving into a competition phases test/gh/win, and then the last 3 months in contest build up take gh, and come in "clean". This is pretty much how its done right now. The athletes can stay hidden away from testing by avoiding international contests, and meanwhile these countries send up an coming younger competitors to international contest who are clean, while their true stars are kept at home to aviod testing.
So, this is how its done, and why the US sux at weightlifting.