It is a very efficient design, but there are operational factors to be considered with such a radical design change. The enormous problem of pressurising a wide flat passenger container as opposed to a long tube (easy- but look at the problems doing that over the years!). Passenger acceptance of the cabin shape and such new appearance, cargo loading difficulties, wing span required....... there are many more 'problems'.
Aero engineering students frequently appear here expressing frustration at the unchanging design scene, but there are reasons for conservatism in design. The Varieeze shape light plane is probably the most efficient design so far, yet it seems to be an aviation design cul-de-sac. Almost all modern design has reverted to basic Cessna 150/Piper Cherokee design. The Avanti is making a small comeback, but look where the Beech Starship went. Innovative design is very unlikely to work in aviation. What succeeds is incremental change only.