Physics and regulations. Cost of fuel is high so you want to fly for efficiency not flight hours. Going super Sonic wastes a lot of fuel, you can go a bit faste

Physics and regulations. Cost of fuel is high so you want to fly for efficiency not flight hours. Going super Sonic wastes a lot of fuel, you can go a bit faster trans sonic for 10%-15 speed boost but the economics don’t pencil out. Besides due to regulations a lot of airline design is stuck in the 60s/70s as redesign to modern standards is insanely expensive.