death to capslock

I could actually see a couple of reasons for this. First, there's the parallel with Harry's two better friends. Then there's that there being two of Crabbe and Goyle sort of gives them more of a personality. Like, they barely have personalities but there being two of them makes them more memorable than just one guy--and it also makes it less necessary for anybody to talk to them because they're a close unit, if that makes sense. If Draco was always walking around with a silent single person you'd expect him to talk more. And thirdly, perhaps most important, Draco walking around with one friend is Draco and a friend/bodyguard. Draco walking around with 2 big guys is like a gang that can make him seem like he outnumbers people. Makes him more of a cowardly bully.