Shit happens.
Space travel is not easy.
I got my new Bose Wave Radio in the mail a few days ago. The CD player didn't work. I had to send it back, and its replacement was delivered a few days later. It is now working fine.
But spacecraft are complicated gizmos. And any little thing that goes wrong has the potential of scrubbing the whole mission.
We've gotten pretty good at designing no-single-point-of-failure systems, but it can still happen.