Flu viruses tend (not always) to come from China due to a number of factors. There are a lot of people there for a start, so, maths. But another factor is the tendency for pigs and poultry (ducks, chickens and so on) to be raised on the same small farms. The virus typically originates in birds, then undergoes a kind of "mixing" process in the bodies of pigs which makes it capable of infecting humans.
This may change to the fact that China is now belatedly starting to consolidate pig farming within larger specialized farms as a result of the mass swine disease that swept through China in 2019. Thus far, however, most Chinese farms have tended to be very small, the legacy of land collectivisation under Mao, which China has been reluctant to change due to concerns about sudden rapid influxes of the population into cities, which could destabilize the country and result in the CCP's rule being challenged. Small farms tend to have a variety of animals knocking about, rather than specializing in one type.