It is quite simple, they had the more recent cleansheet single aisle airframe design with enough ground clearance for modern high bypass engine designs. This ha

It is quite simple, they had the more recent cleansheet single aisle airframe design with enough ground clearance for modern high bypass engine designs. This has baked in a lot of inherent efficiencies including manufacturability meanwhile Boeing leadership refused to invest in a 737 replacement needed in the 2000s.