Well maybe because Dumbledore dicho that the sorting hat takes your choices into consideration - but if he wasn't concerned about which house then i still think he would have ended up in slytherin because of his prejudice against non-purebloods and sense of superiority
On the first book, when Draco sees Harry in the train, and expresses his distastes for muggleborns and blood traitors, Scabbers bit Crabbe, and the three of them fled. Malfoy is cowardly. Same happens in the third book when Hermione slaps him, and also the seventh book, where he had the nerve to ask Harry help to get out of the Room of Requirement. He was never brave, and shouldnt have been in gryffindor.