I got this message from App Store Developer Support (2016-01-02):
Be aware that it can take up to 24 hours for a build to fully process through our system and become available for use. If a build does not finish processing in 24 hours, this can typically be resolved by submitting the build again with a higher build number.
Not much of an answer (nothing about why it can take so long time), but it's the answer Apple is giving us.