Think of it this way and you'll understand the difference. Brake housing is a fully collapsed coil spring, so each turn lays directly on it's neighbor. As suck it's steel on steel and doesn't depend on the plastic to hold it together under compression.

OTOH in derailleur housing the strands are lengthwise, and kept from buckling under compression by the plastic coating. This is fine for derailleurs because they operate at low tension/compression loads, but not suited for braking where the loads are much higher. There's also a safety factor involved since the cable is most likely to buckle under the highest load, and that's when you probably need the brakes the most. (otherwise why'd you squeeze so hard?)