Depending on the timeframe and how many customers they need to email, they do need to stagger sending otherwise they can be blacklisted.
TalkTalk (UK ISP) got public backlash for using the media to announce their compromise via the media before notifying customers by email. The CEO said, the media was the quickest way as sending that many emails would take days.
Well then maybe you should contact them and your email provider and find out why you didn't get the email. From the other comments, you seem to be in the minority on the "finding out about it via HN first" front.
TalkTalk (UK ISP) got public backlash for using the media to announce their compromise via the media before notifying customers by email. The CEO said, the media was the quickest way as sending that many emails would take days.