Paintings from the Tang dynasty (618AD - 906AD) provide evidence of experimentation with different seating styles. The habit of sitting with legs hanging down used to be considered inappropriate but was gradually accepted by society throughout this period. By the 10th century sitting on chairs was fairly common at least among the wealthy. By the early 12th century the custom of sitting on chairs, stools or benches was the accepted norm across all levels of society.
There are probably two main reasons why the Chinese adopted the chair. Firstly the seat was a status symbol, in the early periods being used by the elite. It would have been aspirational for other classes to follow this lifestyle.