NASA chief explains why agency won’t buy a bunch of Falcon Heavy rockets:

"By some estimates, NASA could afford 17 to 27 Falcon Heavy launches a year for
what it is paying annually to develop the SLS rocket, which won't fly before
2020. Even President Trump has mused about the high costs of NASA's rocket.

On Monday, during a committee meeting of NASA's Advisory Council, former Space
Shuttle Program Manager Wayne Hale raised this issue. Following a presentation
by Bill Gerstenmaier, chief of human spaceflight for NASA, Hale asked whether
the space agency wouldn't be better off going with the cheaper commercial
rocket.

"Now that the Falcon Heavy has flown and been demonstrated, the advertised cost
for that is quite low," Hale said. "So there are a lot of folks who ask why
don't we just buy four or five or six of those and do what we need to do without
building this big, heavy rocket and assemble things like we did with the space
station?""

See:

https://arstechnica.com/science/2018...heavy-rockets/