East Asia has an entirely different history of citrus than Europe, and by extension the United States, does. Citrus travelled in different directions, through different climate areas, and so different types were preferred in different climate conditions. Citrus was slow to disseminate across vast distances, but citron had already reached the Mediterranean in ancient times, possibly through the Persian Empire. The orange was next, probably because the fruit had a longer shelf life to be carried by travelers, but had more cold tolerance than pummelo. Initially it was the sour orange, but the sweet orange came very soon thereafter. Although sour orange was known by the Romans, sweet orange didn't really become widely known in Europe until the Fifteenth Century, and orange growing was concentrated in Italy and Spain. Already in the Tenth Century muslim rulers in Spain had planted sour orange trees in their gardens, and by the Thirteenth Century there were orange groves in Spain.
The Northern part of China has always preferred mandarins, hence the name, while in Southern China they have long had "Chinese grapefruit", which is all or mostly pummelo, probably originally brought from Vietnam. In Japan, mandarins (and what are the equivalent of tangors) originally came from Northern China (this probably took place mostly during the Tang dynasty), while Chinese grapefruit came later (less than 400 years ago), from a Cantonese ship from Taiwan.
Pummelo was probably endogenous to the area around Malaysia and the southern regions of Thailand and Vietnam. Mandarins probably originated somewhere in Southern China.
In China, the citron is called "fragrant ball" (this is also the same name which the Ichang lemon is known by in China, although the two are not so much related). The fruit known as Yuzu in Japan is known as "fragrant orange" in China.