Probably because its about nuns. I'm perhaps a casualty of Wars of Religion (not the 16th Century ones, the ones we're holding here in the U.S.). But - feeling like I don't want to watch a film about religion and the monastic life is a dumb reason not to watch a film by the masters Powell and Pressburger.
And it turns out this is a movie about spirituality, and sex. Hooray! This movie is up there with the Red Shoes in terms of sheer stunning visual brilliance - the script is very tightly and economically written. And then there is the remarkable performance of this woman:
Kathleen Byron - who is I think 83 right now and still beautiful - gives one of the 10 best performances I've ever seen on film. At times over the top, yes, but perfectly modulated throughout the film in that it exactly matches the drama, the cutting, the music, the framing and plot, and Oh god, that Technicolor - the best use of it I've ever seen. She's very much the most beautiful monster I've ever seen in what by the end almost becomes a horror movie.
So much has been written about this movie, so I don't want to belabor the points already made by so many others more eloquent than I. Just go watch it on the biggest screen you can to get the full effect of Jack Cardiff's technicolor and Powell's shots of Kathleen Byron's haunted eyes.