This interesting piece on Slate presents the idea that perhaps Bush chose "stupidity" early on as an act of rebellion and has stuck with it even after beginning to get his act together. An excerpt:
Why would someone capable of being smart choose to be stupid? To understand, you have to look at W.'s relationship with father. This filial bond involves more tension than meets the eye. Dad was away for much of his oldest son's childhood. Little George grew up closer to his acid-tongued mother and acted out against the absent parent—through adolescent misbehavior, academic failure, dissipation, and basically not accomplishing anything at all until well into his 40s.