Antibiotics seem to be a part of it, but not the most important factor. The egg naturally has a thin film around it which protects it from contamination by microbes. In the US, the eggs are washed, which removes this film, rendering the egg susceptible to contamination. The eggs then have to be refrigerated to slow bacterial growth. In the rest of the world, the eggs are left unwashed, so the film prevents the egg from being contaminated and no refrigeration is required.