It means that the company is producing fewer shirts than it could be.
The word underutilizing means that the resources are not being utilized or used enough.
The sentence that provides a clue to the meaning of Slaughtered is
"Entire Populations were slaughtered".
Slaughtered means kill or defeat.
Hope This Helps!
I'm not completely sure, but I think it was because of the discrimination that they were facing in Western Europe. They were treated better in Poland.