He is looking for anything those who are now in Crystal Stasis may have had in common, even if it doesn't appear to be unusual at first sight.

He would have to request his Moogle to send the letter to each of them individually, which would involve going through the processing system 50+ times rather than one time only. Needless to say, this is tedious and boring. It depends on whether his Moogle has the time and patience to do it or not.