There’s no good physics reason that I know of. Sometimes, things like this are decided just because of which person or country has the most power to make the decision. There might be a more scientific reason that I don’t know about though.
Well, its not (0,0), which is actually directly south of Greenwich, on the equator. Its latitude is 51 degrees above the equator. Latitude ranges from 90 degrees north (the north pole) to 90 degrees south (the south pole), with the equator at 0 degrees.
Greenwich IS, however, the zero point for longitude, and the reason for this is because Greenwich was where the Royal Observatory was, and this observatory was arbitrarily chosen to be the zero point by astonomers, who could have chosed anywhere else.
This is more a question of history, rather than anything physical or scientific. Greenwich is designated as the synchronisation point for time – in other words, times in places elsewhere in the world are whatever time it is at Greenwich + or – a set number of hours, depending on how far away they are from Greenwich, and whether they are east or west of Greenwich. This is how we divide the world up into different ‘time zones’.
In terms of it being the ‘centre of the world’ – Greenwich is the zero point for longitude, but not for latitude. Zero latitude is anywhere that lies on the equator of earth. The decision for Greenwich to be zero longitude is arbitrary, and was awarded to it as a prize: