People in different countries have different styles of portrait photos. Who can tell me why? Historical factors? Or what tradition?

There are many factors, including historical factors.

Sideways, emperors used to be sideways, such as the king of England, because they painted the king in front of themselves, which was detrimental to the dignity of the emperor. In addition, the positive photos do not reflect the beauty and mysterious courage, especially for women, which highlights the mysterious beauty of sideways. Most parts of Europe are sideways, which may be related to religion. There may be some articles in Jesus' Bible that have played a role. The Mona Lisa is also sideways, which is an art and a sense of beauty.

China is more realistic and will be immortal. Celebrities will know all about history, and their looks will be passed down completely, so that future generations can remember themselves, which is also aboveboard.