Why They Didn't Act

Someone else was supposed to act.
Well, at least they thought someone else was supposed to do it.
True, they could have checked to see if anyone else was going to act but they had other things to do.
Of course, the other things weren't as important as the task at hand and yet if someone else was supposed to do it then it wasn't really their task, right?
And they didn't check because they thought it wasn't their responsibility in the first place.
So you might say that convenience trumps importance.
Doing what was important was out of their way.
And whatever is convenient is always their way.