When people created calendars many 1000's of years ago, they were not able to get to most other countries. Transportation and communication was very limited. Different countries would have their own language and time for the new year, depending on how they measured things. They had no knowledge of what other countries were doing.
We measure the new year based on a calendar and the sun, but China, for example, uses the new moon, which can vary each year. The traditions have stayed and that is why there are different times countries celebrate the new year.
