Expanding the International Space Station is a major project.
First of all, the ISS is an international collaborative project, and it truly lives up to its name.
While NACA has a profound foundation in deep-space exploration and astronautics, it is not adept at the construction of large-scale in-orbit spacecraft.
The reason can be traced back to the last century's space race; after America's victory in the Apollo program, it became somewhat complacent, not caring much about launching just a few modules into low Earth orbit.
Of course, it's not that NACA had no interest in long-term manned missions in low Earth orbit—in fact, this project was already included in the Space Shuttle program.
