Look at it this way: the drag is at its minimum when both main wing and horizontal stabilizer have exact same AoA which is the best L/D AoA for both. But this has neutral stability: if you pitch up from best L/D AoA to stall AoA, both wings reach stall AoA at the same time and the plane mushes down without dropping nose and recovering from stall. A plane is stable when the rear wing has lower AoA: when the forward wing stalls and drops, the rear wing is not yet stalled and so the nose drops. If the rear wing has higher AoA then the plane is negatively stable, and the rear wing stalls first, dropping the tail and pushing the front wing into stall.
The way to get positive stability is to get the CoG ahead of its rear limit (where the stability is neutral) so that the front wing carries more of the weight and rear wing carries less or actually pushes down - but since the wings are not at their best L/D AoA then, this adds drag.