The US took a more active role simply because it was able to do so.  Up to and for many years after the Secession or Civil War in the 1860's, the US foreign policy was defensive -- designed to keep other outside influences, mostly European, out of North and South America, as most of the country's energy was put forth in expanding west.  However, after reunification and the settling of the West, the US began to expand its influence internationally and take a more vigorous, if not offensive role.  The Spanish-American War of 1898 has proven to be the turning point from a defensive to offensive foreign policy -- By declaring war on Spain, the US sought to influence and perchance establish statehood in Cuba and the Philippines.  In the first few years of the 20th century, the US influence in Central America paved the way for the Panama Canal, and its influence in Mexico brought about questions of Mexican autonomy.