Subjunctive after parce que

I have come across the following sentence in a novel that I’m reading: ‘On avait laissé dans un coin une applique allumée, parce que la lumière des bougies eût été insuffisante.’ I would like to know why the subjunctive is being used here, because I have always been told that parce que is followed by the indicative. The novel in question dates from the late 1940s, so it’s not a very archaic usage.