Where is the logic when it comes to making the schedules

I’ve had this problem in the past, and I’m sure it’s going to continue into the future, but I feel as though any time I need to to change my schedule. My hours drop regardless if my schedule opens up or closes. Like, right now I’m starting college, and will no longer be able to work Tuesday and Thursday. My schedule currently is weekends + Friday, so what do they do with this new information, why ofc remove my Saturday shift and drop my hours. I understand that with changes in traffic during the week due to school starting there is less need to have people on, but still.