The Earth is a globe, and the sun rises in Australia, then India, then Europe, then America, then the Pacific Island, then Australia again. Somewhere on this circle, we have to pick a north-south line and call it the place where a new date begins. It would be awkward if this "Date Line" line went through a major population region: people on one side would be in June 15th (for example), while people just on the other side would be in June 16th.
The Earth has a convenient place with few people: the Pacific ocean. Plus historically, people in western Europe wanted to be as far away as possible from the date line. They wanted the colonies in America and Australia, and India, to be in the same date as England and Spain as much as possible. Since western Europe is about as far as possible from the mid-Pacific, the geography works out to favor the mid-Pacific as the place where the date changes.
Australia is on the west side of the mid-Pacific, so the date changes there early. The US is on the east side, so the date changes there much later.
If China had colonized the Americas, southern Asia, and Australia before 1450, they might have picked the mid-Atlantic for the date line. They did some long-distance exploration, but pretty much stopped after 1433 as it was too expensive and they had problems back home.