Why aren't the Chinese and Russians successful at building airliners? International politics aside, they have workable products, so why can't they break into the duopoly.
Not so, not for Russia anyway:
There are other commercial aircraft manufacturers but essentially in each size segment you will only see two players. Embraer doesn‘t make any direct competitors with Airbus or Boeing, as an example.
Them selling out to Airbus had more to do with general issues at the parent conglomerate (they also exited trains and snowmobiles to raise cash). But they still make corporate jets.