Why is the International Date Line not straight

Why is the International Date Line not straight like the Greenwich Meridian - here is a concise answer:

http://en.wikipedia.org/wiki/International_Date_Line

An extract from this page: "For a large part, the International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. However, because the date to the east of the line is one day earlier than that to the west of the line, the line deviates to pass around the far east of Russia and various island groups in the Pacific. Thus, the two largest deviations from this meridian both occur to keep the date line from crossing nations internally."

A great time zone map:
http://www3.shastacollege.edu/dscollon/images/Figures,%20Diagrams%20and%20Maps.htm