Anti-missile defense systems on the ballistic scale are pretty much a 50/50 proposition. There's signs they may have been testing multiple re-entry vehicles as well, which makes it even more difficult.

But just because anti-missile systems weren't used here doesn't mean they don't exist. US/Japan could have decided the missile was probably headed for the Pacific and decided not to intercept. The location and capability of US anti-missile batteries would be strategically useful information for NK, so better to keep them quiet if you don't have to use them.