You'll also note that they tail airfoils were simple flat wedge shapes with very knife sharp leading edges. It's all got to do with how the air flows at hypersonic speeds. At subsonic speeds the X15 was a drag bucket that barely could hold itself aloft but at over Mach 1 things started to work a lot better. The shape of the fuselage, wings, tails and airfoils were selected to work at multiple Mach numbers where the shock waves are a key component of both the overall planform and airfoil design. The design is wholly integrated from nose to tail so that all the shock waves work together.

Howzat? Did I pass?