Because the software didn't cause it.
Look, by all accounts the software was/is a piece of piss, but what made it such an egregious scandal is how the Post Office leadership dealt with things. There was really no good reason for that to happen. They just ignored reports of problems (proper reports written by auditors, not vague rumours). They lied to postmasters by saying that no one has problems (when, in fact, there were hundreds of people). Lots has been written about all of this and I won't repeat it all here.
So I must object to the phrasing of "caused by their shitty software". Of course lots can be said about the failings of the software itself and Fujitsu also lied and covered their tracks so they are not entirely blameless. But they emphatically did not "cause" any of this: it was the Post Office leadership who primarily caused this mess.
Lots of things go wrong in the world, lots of things are defective. What often matters the most is not so much the mistake or defect itself, but what the response to that is.