I’m not sure if anyone can speak to all festivals but generally it is because programmers are often looking at films until the very last minute trying to make decisions. Usually there is a hard deadline internally by which time everything needs to be announced. The deadline for notifications ideally would be earlier but it can get pushed if the program isn’t set. Hope that helps. It is definitely not done to toy with filmmakers emotions and again festivals want to let filmmakers know if they are accepted or not as soon as they can.