The real question is: why don't they try to contact the recipients during the 24hr period prior to the announcement, at that recipient's acceptable local time zone?
I don't think a Nobel recipient is going to spill the beans on the announcement.
It's a bit unfair to ask someone to keep their mouth shut when they've received one of the most prestigious awards possible in the world. Given that, it's reasonable to assume they'll share with _someone_ during said embargo and once that cat is out of the bag you might as well assume the news will leak. Hence the curtesy call ahead of the general news breaking just a bit beforehand.
I don't think a Nobel recipient is going to spill the beans on the announcement.