This is because Japan is the most valuable of America's 58 treaty-bound allies in two ways. First, the United States cannot maintain global preponderance without Japan. Second, in military terms, the alliance with Japan is the most symmetric of America's alliances. Japan meets and surpasses various conditions for a power-projection platform. One is industrial and technological development.
Few allies are capable of repairing technology-intensive weapons of the U.S. armed forces.
Another is economic capacity. In the course of acting as the world's leader, the United States sometimes finds itself in a financial shortfall. While not discussed often, the public's friendliness toward the United States is another important condition. From the U S. perspective, the Japanese are exceptional on this count, even among allies. The loss of the Japanese archipelago would be as damaging to the security and prosperity of the American people as an invasion of the U.S. homeland.