This was the result of a deliberate strategy from the 1920s on. Hollywood studios lobbied the US government to promote their movies and to give them favourable treatment in trade deals.

The movies were carefully crafted to appeal to foreign markets and there was prior consultation with the censors in major markets like the UK to ensure the movies would not run into trouble. If necessary scripts were amended to remove derogatory references to other countries or similar problems.

There were foreign language versions of most films. After the coming of sound the big German and French studios were at a disadvantage. British films were under such pressure that the government introduced quotas of domestic films that had to be shown in every cinema.

The fact that US films were designed to be commercial successes with glamorous stars, aspirational themes and huge promotional campaigns certainly helped.

Source:

Ruth Vassey, The world according to Hollywood