Boeing has generally been one of the lynchpins of China-US trade relationship, and with both the current geopolitical climate and the ongoing quality issues of Boeing aircrafts, of course the Chinese airlines would want to hedge their bets and take a risk on domestically produced jets, even if it is new and relatively unproven at this point. It doesn't mean that they will stop buying from Airbus or Boeing completely, though.

The C919 uses many American/European produced components/subsystem, most notably the CFM LEAP engines, which is also used by both Airbus and Boeing.