To keep you from subscribing to a particular channel for one or two events and then subbing to another for one or two events every month. I'm sure Dish, and every other supplier, pays HBO and the like a monthly rate per sub. When you sub to HBO for those two events and then Showtime for anther two events thye have to pay each one for the whole month but you paid just the one fee.......
Because it can make your bill more confusing. As in if you keep calling in to make changes all the time or doing it online. They used to charge you anytime you made a programming change no matter what timeframe.
Potentially a lot of customers would be changing the programming every so often it would be a chaos. The current system is better than the previous one.