It’s that:
a) NASA didn’t/doesn’t have the capacity
b) The Russians did have the capacity but it was expensive and also painful to American egos to utilise this repeatedly, not to mention a risk due to geopolitical uncertainty.
c) NASA therefore funded Boeing and SpaceX to develop ‘home grown’ capability. (Note: SpaceX received less than Boeing for this development.)
d) Boeing’s space division lost up a big lead (over SpaceX) through poor development and execution, echoes of which continue to this day with the issues seen with Starliner leading to the two astronauts being effectively ‘stranded’ for a period of
e) SpaceX did a good job of developing Dragon and getting Falcon 9 rated for human flight. Since then, their reliability, low cost, and high launch cadence, coupled with Boeing’s missteps, has led to multiple contracts and successful launches.
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It’s also worth mentioning that SoaceX was started when Musk wasn’t so crazy (remember, once upon a time, he was the woke darling of the progressive center-left) and also that a lot of its success and stability may be thanks to its COO, Gwynne Shotwell. Calling it Musk’s “toy company” ignores the huge advances it has made, thanks to the fantastic people that work there, and also (gulp) Musk himself.