In November 1915, Einstein describes Sommerfeld the three reasons, why he finally gave up on the Entwurf Theory.
He didn't succeed in interpreting the forces of inertia in a rotating system as the effects of a gravitational field. The theory produced the wrong value for the perihelion motion and he failed in his attempt to deduce the theory from mathematical principles.
Text__vGpoQKOcEd-WfYQ0GneCwQ_title=Letter from Albert Einstein to Arnold Sommerfeld