Why was the Prime Meridian established at Greenwich

In 1675, the Royal Observatory at Greewich, outside of London was created to study the determination of longitude (a vital comonent to oceanic navigation). The United Kingdom and, by association, the United States, used Greenwich as the reference point for longitude.
Thus, by the time of an international conference in 1884 to establish fixed degrees of longitude on the planet, many countries had been using Greenwich as the Prime Meridian for decades so the tradition became internationally accepted