Why is the prime meridian located where it is?
That was strictly a matter of international agreement. Unlike the equator, which is
defined in terms of the shape of the Earth, the origin of longitude could have been
anywhere that everyone agreed on. England had the largest worldwide empire, and
the longest seafaring tradition and history, and the Royal Observatory in place to
verify and monitor the measurement, and took the initiative to organize the
international conference to set up the uniform, global navigation reference system,
where it was agreed that the meridian through the Royal Observatory in Greenwich
would become the zero reference for longitude.