It was never required as a tradition. Of the first several presidents, only Washington even served in the armed forces. In modern times, William Taft Woodrow Wilson, and Herbert Hoover did not serve in the military.

Military service was simply a huge political boost. Soldiers like Grant and Andrew "Stonewall" Jackson used the popularity generated by their military successes to launch political careers. During the 2nd half of the 1950s, the constant state of warfare (real or cold) generally meant the American public preferred candidates with military experience over those without. Once the Cold War ended, that preference died.