Because, I looked through last year's postings around the same time. My conclusions first, and then the data. (Of course I am assuming all posters are truthful 100%)
1. Last year the first batch of emails went out on April 21, 2016 Thursday. Some people indicated that they got the "alternate list update contact info" email on that day.
2. On that day and following few days, there were folks who complained that they did not get any such "alternate update contact info" email.
3. Some of them got it on April 25, 2016, Monday.
4. There were others who were waitlisted but never got the "alternate list update contact info".
So, the alternate list update info email is sent in batches. Some people get it earlier than others. Some don't get it at all. The update contact info has one purpose. These are potentially people who will receive an offer after April 30th, when SLU finds out from students who are withdrawing. When students receive the email from SLU, people have a short window of time (1 to 2 days I think), to accept or they lose it. So they want the students to provide an email which they watch regularly not once a week or once a month.
In any case, one of the ones that did not get it called SLU and posted what they said. That as long as you do not withdraw from the waiting list, you are likely to be considered. However, it seems logical to me that if you have a good chance of receiving an offer, they would want to give you the opportunity to provide them a contact information that you access frequently.
Now, here is the data. This is verifiable by looking through this link:
2015-2016 Saint Louis University Application Thread
Alternate list update contact info email went out on april 21, 2016.
mbio2015, postiveprotons, Bermie, got it in April 21, 2016.(last year it was a thursday)
Bermie interviewed in February, placed in Alternate list in March.
Interestingly, more students got update contact info on April 25, a monday as well. (If SLU wanted to make a single contact list and send a single email, they could have just as easily)
eartwj and sweetdisposition and Bboyzen202 got that on April 25 monday.
swolesoul did not get one until april 24, sunday. meow+or_never also did not get on April 25 nor did believer2015
ac909009 did get an email asking if he still wanted to be considered. might be the same as contact update.
Bboyzen202 got it on April 25. (Interviewed in December). Note Bermie got on April 21, but interviewed in February.
(So the update contact list request is not sent out in the order of the interview date. Most likely it is in the order of the ranking in the alternate list).