Why did the middle and late Tang Dynasty prohibit Han people from marrying foreigners?

In 779, the fourteenth year of the reign of Emperor Zongli, "there are often thousands of Uighurs staying in the capital, and twice as many people living in the capital." In response to this situation, the Tang government issued an imperial edict, stipulating that "anyone who is returning to Hudu should be given it."

Clothing is not valid Chinese. "It is strictly forbidden to seduce Han Chinese women for wives or pretend to be Han Chinese in any way. At this time, in Guangzhou, foreign residents still live with the Han people, without apartheid and enjoy free marriage.

Live a happy life. In the first year of Wenzong (836), when lujun was appointed as our ambassador to Lingnan, he forced foreigners to live separately from Han Chinese, forbidding intermarriage and forbidding foreigners to occupy land and build houses. Book House Yuan Gui Volume 999

In June of the first year of Kaesong, China was ordered not to communicate, buy or sell, get married or associate with foreigners in private. It shows that there is a limit to the freedom given to foreigners to marry Han people in the Tang Dynasty.