Because of regulatory capture, any competitor will have to be even _more_ subsidized than Boeing is.

A few years ago, Bombardier introduced a new plane, the C-Series. Even though it did not immediately compete with Boeing's own offering, Boeing successfully lobbied for an extravagant 300% import tariffs to be be applied which killed the primary market (US) for it. Canadian government folded and essentially gave the otherwise technically successful project to Airbus in the hope of salvaging any industrial returns on eventual mass production. The airplane is now known as the A220.

Any would-be competitor will not only be fighting Boeing, but the whole US government.