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.