Why is English the main language of the U.S.?

The rulers of any lands would force their languages upon the populace. So whoever was a ruler at any given time made their language the primary one to use.
Oddly enough, the most famous exception to that practice was in Biblical times. The Babylonian Empire, at its peak, ruled a huge swath of land that extended far beyond the Babylon city limits. Many languages were spoken there. The language of the Aramaeans, for reasons I've never seen analyzed, was spreading as the lingua franca of the empire, and ultimately became just that. Ironically the Aramaeans themselves vanished into the "melting pot" but their language continued to be the common tongue of the region, right up into the era of the Ottoman Empire.

It is still spoken and there are even websites in Modern Aramaic. But Arabic, Farsi, Persian, Azeri, Kurdish, Armenian and the other national languages of the Middle East are gradually reducing its influence. Since it is well recorded it will never be forgotten, but many of its speakers admit sadly that their great-grandchildren will probably not be able to speak or understand it.