Story

From a layman's perspective:

Boeing is Boeing. They know how to manage complex technical programs .

SpaceX is SpaceX. They were the first commercial suppliers of the space station.

SNC is ???. As far as I'm aware they haven't built a complete craft like this before anywhere near this scale. They've built parts and engines, but not the whole kit and kabootle.

I'm sure by granting Boeing they were appeasing the more risk adverse people at NASA, and by going with SpaceX they appeased the more innovative faction. I don't know if SNC had anyone emotionally tied to its proposal. Not that "emotion" was a criteria. But we'd be lying to ourselves if we were to pretend that emotion had no role at all.