Complex sentence: In the United States conditions-a large land mass, uniform language and culture, large and growing population, and good flying conditions-favored rapid growth of private airlines and many regional airlines grew and looked to expand traffic to Canada and Latin America.
Split into simpler sentences: In the United States, the conditions of a large land mass, uniform language and culture, large and growing population, and good flying conditions, favored rapid growth of private airlines. Many regional airlines grew, and looked to expand traffic to Canada and Latin America.