I don't know why it has changed but it is a good question. When I had my studio in Ohio, I kept regular business hours and fully expected same day sessions. If the day had been slow, I'd just spend the night and be ready for 2am calls. I laughingly called it my after bar business when the phone rang. Interestingly enough, when I moved to Providence, Rhode Island, my studio was considered more of a designation spot that people planned for and traveled to. I had less hourly and a lot of extended. I tried same sessions in New England but it didn't work like it did in Ohio. I've got more questions than answers on the topic of the shift. I do wonder if short notice sessions still reign in bigger cities.