As far as I know it's the business class section that makes the money. Business travellers want choice of departure times.
Economy (leisure) travellers don't want to board a 7 hour flight at 3pm. They want a 10am departure because they want to get there, check in and go and eat etc. Business pax want to arrive at 10am not depart. You want to operate bigger aircraft with fewer slots you have to alienate either the economy pax or business pax. I know which ones I want to keep.