I assume this has already been mentioned, but I feel like it's probably the cards and the plastic molding in the Core Set that are making that more difficult than usual to produce quickly enough to keep up with demand. By including the cards and whatnot, that's probably complicating production over the book alone.
I would expect the opposite, given the immense amount of lead time book production typically needs, and that's before all the international shipping and trade complications that have been added in the last year or two. Still, I'd wager the cards don't help much, which makes me wonder what the production time on their upcoming kickstarter will be too.