The Greenwich meridian is the point from which all time is measured because, when time zones were being set up around the world, the British Empire was the dominant political force in the world. Because it was the most important country in the world, it got to set the line from which time was measured.
The Greenwich meridian is the line that goes through the Royal Observatory in Greenwich, which is a district of London. This was the main British facility for astronomical observation. In the 1800s, Britain was the world’s dominant country much as the United States is today. Britain was the center of many things, including science and commerce. Because it was the center of science and commerce, it got to establish itself as the center of things when time zones began to be established.
There is nothing special about Greenwich that makes it the correct place from which to measure time. Time is only measured from there because Greenwich was where the Royal Observatory was located at a time when Britain got to set the standards for the world because of its power and wealth.