Why do you think European powers established spheres of influence in China rather than colonies, as they did in Africa and other parts of Asia?

The answer in this case is a simple, but often overlooked. Unlike most African areas that Eurpeans had colonized, or the Americas, China was a strong unified country with a developed military. While they were not as advanced as the Europeans they were still a threat and with such a long supply route that would have to have been used if war ensued there was no practical way to fight the Chinese at that time. Also like in Africa if one country were to really push to take territory other European countries often oppoesd or pressured them. I believe that if war did ensue between a European country and China in order to make CHina a colony oher European countries would have gotten involved to stop the aggressor from gaining territory. That said; in the case of the opium wars the British were the premier naval power of Europe and were not fighting as aggressors to take territory, but to hold onto their trade routes. They instead did recieve Hong Kong in the end of the wars until recently in our history when it was returned to Chinese control.