I think that they use the "soft" cable to make them more flexible and so less likely to get caught on branches and other obstructions. They could use a thicker outer sheath, but I think that the blades would cut it just as easily or they could use an armoured cable, but the would be very stiff and difficult to use. At least with the soft, flexible cable the blades tend to cut straight through and sever the whole thing, where as a thicker cable might just get damaged with a small cut which is not so obvious and could be more dangerous. Which ever it is it just shows how important it is to use a circuit breaker.