(This may be an anti-answer.) Why is difficult. It's just how air flows past one obstacle or another: something that a wind tunnel can put numbers on, but not give an eloquent explanation for. Amuse yourself with debates about the Coanda effect, which devolve into anthropomorphism.
For a given definition of efficiency, one combination of planform and washout and sweep and taper and camber and vortex generators and aerodynamic elasticity and who knows what else will outperform another.
For the famous Spitfire case, everything else being equal, an elliptical planform yields, per unit wingspan, more lift and less induced drag, over a wide range of angle of attack, than any other planform, rectangular (cheap to manufacture) or otherwise.
Theorists writing "consider a spherical cow" papers attribute this to the downwash being constant spanwise, so each spanwise sliver of the wing is working as hard as it could be. (See what I mean about anthropomorphism.)