He probably realized early on that it wasn't in his best interests to, and so just turned that part of his brain off. Curiosity seems like a dangerous thing in North Korea.
People's sense of self-interest is a strange strange thing.
I was wondering about the same thing. Could it be a self protecting strategy? In the article, it mentioned that the Kims see him as an outsider, and by continuing speaking to them via a translator, it kept the sense of him being an outside for the 20+ years.