Because that's where the (British) Royal Observatory was.
Greenwich, England, was chosen as the prime meridian (0 degrees longitude) at the International Meridian Conference in 1884 due to its long history in navigation and cartography. The Royal Observatory located in Greenwich was already a well-established center for astronomical observations.
0 degrees longitude is called the Prime Meridian. It runs from the North pole to the South pole, passing through Greenwich Observatory in England. Greenwich was chosen by international agreement (largely due to it being the de facto standard anyway), though historically other conventions have been used.
It was chosen because, at the time England was the biggest country and superpower at the time. So the agreement was to run the prime meridian through Greenwich. It is also the closests to the middle of the earth's sphere.
The Prime Meridian, located at 0 degrees longitude, is the imaginary line chosen as the starting point for a day on Earth. It runs from the North Pole to the South Pole and passes through Greenwich, England.
It was chosen as the base 0 degree meridian for longitude.
It was an arbitrary decision to locate the Prime Meridian at Greenwich because that is the location of the Royal Observatory. Every point on Earth exists on a specific meridian. Any one of them could have been chosen. The decision to use the meridian at Greenwich, England was made during the International Meridian Conference held in 1884 at Washington, DC, USA.
The Greenwich Meridian is also known as the Prime Meridian. It represents zero degrees longitude, although it was chosen arbitrarily as the zero marker.
The Prime Meridian runs through London because it was established by Sir George Airy. The choice of location for a prime meridian is entirely arbitrary, so a British scientist chose a line passing through the astronomical observatory at Greenwich in London.
By an election.
England's Nickname are the Three Lions
England won't have king until the present queen dies.
Until Greenwich was established as the site of the Prime Meridian, the maps and charts of the various countries and peoples would reckon longitude from the meridian of some major place of their own. It would be London for the English sailors, but for the French, it would be Paris and for the Italians, Rome or Genoa or Venice. At the time that cartographers and navigators were feeling the need for a single international standard, Britannia was definitely ruling the waves and leading the world in the sciences of exploration. The Royal Observatory, in an old deer park above the town of Greenwich, seemed the fitting place for the Prime Meridian.