I've done some of these mail order ant farms with my boy before. Honestly, ants die - they seem to have a pretty short life span in general. That's why queen ants lay hundreds (thousands?) of eggs a week. If you want the colony to endure, you will need to get a queen so the ones that die are replaced. This adds an additional part of colony life to observe, as well, because you'll be able to see the egg laying, larval stages, etc.. Most of the places you can buy the ant farm from will send you a couple vials of ants, and then if you pay extra they'll send you a queen to put with them.
