Probably an answer which is less to do with Physics and more to do with politics:
Greenwich Mean Time (GMT) was adopted as the world’s time standard at the Washington Meridian Conference in 1884. This conference also established Universal Time, from which the international 24-hour time-zone system grew. This is why all time zones refer back to GMT on the prime meridian. The prime meridian at Greenwich, in the United Kingdom, has served as the reference line for GMT since the late 19th century.
(see the reference for full details, nice explanation. Hope this helps :)