I'm guessing it is the heat resistance of the missiles leading edge material. Hypersonic cruise missiles all seem to attain Mach 6 and below. The boost-glide weapons fly faster but up much higher. I think the Russians have ABMs that make it to Mach 10, but that is also intended for high altitude intercept. The X-15 attained about Mach 6, again up around 100,000 feet.