Antares has a maximum payload of 6000kg. This is roughly the same as the launch mass of the Apollo command module, which weighed 5500kg. To rescue the command module in an emergency, Apollo missions had a 4200kg launch escape system. So we might expect a escape rocket to cost Antares almost half of its payload capacity.

Furthermore, adding a launch escape system would mean adding parachutes and flotation to a payload which otherwise doesn't need it.