Mostly because of some threads I've been reading in various places. I think that there is an outside conception that religious homeschoolers use a lot of religious curricula, or only use the Bible to home school, or that they are sheltering their children from the full sweep of ideas. I'm not sure exactly where this idea started- maybe it was more true earlier in homeschooling but in my experience this is less and less true.
Personally, my family qualifies as "very religious". My husband is a pastor, after all, and a conservative one at that. We're probably on the strict side of the spectrum but looking at our curriculum choices it's mostly secular.
The only religious program for a core subject, or really anything other then religion, is Prima Latina- a Catholic program for Latin.
Everything else is secular- from the history to the science to the LA products. And it's mostly on purpose. Some of it is my experience with reading through religious curricula for history and science is that the publishing group's biases and views leak through in sometimes insidious ways. I don't want to have to run everything through a filter to see if we agree with it or not- if I wanted that I'd be devising my own curriculum. And I'm not.
Amusingly, this was one of my thoughts when we were first seriously considering homeschooling- only from the other side! I thought it'd be nice not to have to deal with the public school bias. And now I've discovered that there are biases all over and it's nearly impossible to get away from it.
Susan Wise Bauer has written an interesting post about this here: http://www.welltrainedmind.com/neutral.php