alt Hacker News"Or why it would be?"
In my part of the world, it's definitely declining church attendance. If you don't have a huge population of young boys and girls trained in those choirs, and instead the population were self-selecting into voice training, I would definitely expect a serious sex imbalance.
What part of the world, may I ask?
I've found some excellent vocal groups from parts of Asia not known for being especially Christian, but it seems that choral/a cappella music is very connected to Christianity there too.
I believe church attendance is getting more gendered too, with a shortage of men.