There are many different reasons people from developed countries would want to immigrate to the USA, it will vary from person to person.
For me personally, there are a few reasons:
1) Career opportunities - I've studied media and I'm particularly interested in going into film/television and the opportunities here for this industry are severely limited.
2) Culture differences - The culture of the U.S. and Australia seem very similar from the outside, but they're not. At all. Warning: I am talking in generalisations here. In Australia, we like to bring people down when they're getting 'too' successful. In the U.S., peoples genuine fortunes and successes tend to be celebrated. In Australia, 'dreaming big' and 'hope' aren't really a thing here, whereas those are two fundamental things that help most people get by in the U.S., even when things may seem hopeless.The U.S. is a leader in innovation and creativity, Australia is not. Etc, etc, etc.
3) The people - I find Americans to be a lot more open and friendly than Australians. Some Australians have a tendency to be quite 'closed off.' To strike up a conversation with strangers here is not really socially acceptable, whereas in the U.S., I spent a lot of time travelling alone and people would would see that, hear my accent and would be interested to know more about my story. I really don't think we have that same kind of acceptance of outsiders and genuine interest in other people here.
4) Australia is TOO expensive!!!! - This is probably the least important part to me as I want to move to a city in the U.S. that's equally as expensive, but it's a big deal for a lot of other people. I generally don't mind paying a bit more if the price is warranted, but it's usually not. I don't know how many times in Sydney I've paid way too much for a subpar meal with even worse service.
5) The US is simply a more exciting and interesting place - Australia is a beautiful and comfortable country, but it can also be very boring. It's like living in a bubble disconnected from the rest of the world. But mind you, lots of people are very happy living in that bubble - it just depends on what you want with your life.
We are isolated. You have to travel yonks to get anywhere interesting. The U.S. is very unique in that the country is so different. You can travel from Nashville to New York and feel like you're in two completely different countries. You just don't get that experience here. Going from Brisbane, to Sydney, to Melbourne, it's hardly any different. Everything is just on a bigger and more exciting scale in the U.S.
Mind you, I'm happy that I was born in Australia. I am fortunate and grateful as I know there are many people who would do anything to come here. Australia has been a terrific place to grow up as a child and a teenager (incredibly comfortable and easy lifestyle), but ultimately, now as a young adult I don't feel as if life in Australia is satisfying anymore. I hope this could be of some help to you.