It seems intuitive to me that if you're maintaining such a clean room the only bacteria that are going to live there are ones hardy enough to survive in those extreme conditions, you're just going to be running an accidental breeding program for bacteria likely to survive on a spacecraft.
As opposed to just assembling all this stuff in a room open to the elements. It would then be completely inundated with bacteria, but none of them would have any selective advantage in being hardy. You could even feed them on purpose by spraying food for them everywhere.
Then when you launch the spacecraft those fat comfortable bacteria would all instantly die because vacuum isn't an environment that's anything like what they've had to deal with, unlike the hardy ones in the clean room, and the few survivors would have no time to develop hardiness. They'd all die within hours from launch.