I don’t remember exactly but it probably had to do with the logic speed back in those days. The cpu updated screen memory with BitBLT (an operation copied from

I don’t remember exactly but it probably had to do with the logic speed back in those days. The cpu updated screen memory with BitBLT (an operation copied from the PDP-10, hence the name) and the screen was allocated 2/3 (!!) of the memory bus speed. So they ran the logic as fast as they could and that was how many dots they could afford. Remember the Alto was the origin of bitmapped displays.