The SRS lists are tied to the emails. If we generated a list of every single user with every single language they're studying, it would put a huge amount of stress on our servers. This way, it's only a fraction of that since many users have created LingQs in multiple languages but have turned the Daily LingQs off for all but one or two.
For the emails that weren't sent, we're looking at adding these lists to the Vocabulary section so even if you missed the email, you won't have missed the list.
@mikebond - I recommend setting up a filter in your email client to have these LingQs of the Day emails automatically sent to a separate folder. That way you'll only see them if you're actually looking for them, but your lists will still be accessible through the Vocabulary section.