I've been thinking about this since the other thread too. There's definitely life circumstances, and personality traits, and other non-writing projects and passions that the writer pursues. There's also the way different people approach things. I know I personally can be very creative and insightful, but I need time. I can only get to something creative and insightful if I've actively thought about the topic, then set it in my subconscious to marinade for some period of time, then come back and thought about it again. When good ideas strike me seemingly out of nowhere it's really because they'd been marinating. Other people can be faced with that same topic and come up with something creative and insightful on the spot. (I am very jealous). Novels for me are hundreds of these things, that have to get their time to get to their best point. Then I have to work through them and explore them and put more things aside to simmer. Even with perfect life circumstances I'm not going to be able to write anything good fast. I don't work like that, in any aspect of my life.
I also think it matters what kind of stuff one is writing. Most of my personal favourite writers averaged probably 2-4 years per novel, often plus short stories (which seems prolific enough to me!). My favourite writers fall more into the "literary" camp, with character driven novels full of psychological complexity. I think it would be very difficult to "churn out" those kinds of books, no matter your personality and circumstances. I consider Margaret Atwood tremendously prolific for an internationally acclaimed literary writer, and her average seems to be one book every 2-3 years (though she also writes tons of poetry and non-fiction, so maybe a bad example that way).