Draco absorbs some information like a sponge. He can memorize facts and dates and potion ingredients and spells as good as Granger ever could. But other things, the things that really matter, hit a block in his mind, and he refuses to learn at all.
Draco refused to learn that it was alright to have doubts. All through the war, he would not admit that he was anything less that certain. Whenever that tickling feeling of doubt crept into his mind, he would immediately do his best to squash it. And he never learned.
Draco refused to learn that it was alright to feel fear. He never understood, and wouldn't understand, that fear was essential to being brave. If Voldemort's presence, if the actions taking place, if what he was ordered to do made him feel slightly uncomfortable that was one thing. But to feel fear? Unacceptable.
Draco refuses to learn that it is alright for a leopard to change it's spots. And perhaps that is the worst thing to never learn of all.