It's a somewhat puzzling change, but it's not something that has much relevance to most fans or even fansites (since the new sites retains the old directory structure any broken links are easily fixed).
What was interesting is how it seemed to happen gradually; The Reference Book was moved first, but some other subsites like the general "About Digimon" page (which, come to think of it, I haven't seen fully translated anywhwere yet, curiously) were still on the old domain, but by now it seems everything has moved over.
Who knows why it happened, maybe it's just some new company policy about where and how to present their franchises, a very tiny optimistic part of me hopes that maybe they vacated the domain because they have plans to put something completely new there and this is their way of archiving the old site or something. But we'll see. It's not much of an issue either way.
