A Google News publisher asked in a Google News Help thread why his publication rarely comes up in the Google News alerts for specific queries he subscribed to. Inbal from Google explained how alerts get sent and how you need to rank in Google News to be included.
In order to be included in Google News Alerts, an article must be included in Google News and appear in the top ten results of Google News for specific search terms.
Clearly, the first part of the answer is obvious. To be in Google News Alerts, you must first be indexed by Google News. The second part is less obvious for many publishers, but for SEO's it likely makes a lot of sense. You must be in the top ten results at the time the alert is triggered (both RSS and Email) for you to show up in Google News Alerts. I believe the frequency of your alerts makes a difference, because the Google News rankings often change throughout the day.
Forum discussion at Google News Help.