Because in the US you can. Everywhere you go in the world there are a very smart individuals.
What makes one country more innovative than the other is its ability let smart people not only express their ideas, but also give them tools to easily turn their concepts into products and services.
Society plays a huge role as well. For example in America it is ok to fail at being an entrepreneur, or to fail as a researcher who spent 10 years trying to find a cure for a disease. No one will laugh at you or tell you you wasted your time. In fact people will admire you for your courage.
Money is the third biggest factor. In the US the private sector invests in just about every damn thing. And if you are lucky or incorporate your company properly you do not have to pay back one freaking penny may you fail to turn your idea into a profitable company.
That is why America is innovative. Capitalism at its realest sense.
