crispy cable! Why wouldn't a breaker trip?
Assuming the cable in question was on an AFCI breaker, I like to think that it would have tripped, but who knows?
If a cable develops a fault due to being nicked, pinched, stapled through, whatever, that fault can have a high enough resistance not to trip the breaker. It might, for instance, only be drawing an amp or two, much less than it would take to trip the breaker. It will still generate a good amount of heat at the fault, though, which will burn up the cable.
If a wire gets broken in the cable, it could also develop a series arcing condition, where current through the circuit is actually having to jump a small gap along its path. This would also create some heat, while the circuit may seem to be operating normally. Since there'd be no more load on the circuit than whatever was plugged into it, a breaker wouldn't trip. This is one of the conditions that an AFCI breaker is designed to detect.
So which type of breaker was used on this circuit? Regular, or AFCI?