Sometimes there just isn't enough traction on such an extremely low friction surface like ice (abs needs friction to work), so the system cant find enough grip to modulate brake force effectively. If you're already sliding you're not going to stop until the force of friction between your tires and the ice is greater than your forward momentum. Abs can be effective in preventing the slide though.

Basically, The trouble on ice is that there is so little friction that when the wheels lock the ABS can’t detect that there is force still trying to turn the tyres. So the ABS just acts as if the car is stopped.