Because the uppermost layer of ice has a thin layer of water on it.
Ice is like an extremely thick stack of newspaper. The top most layer of the ice we move on is ‘loose’ compared to the lower layers. Lower layers have each other to stick to. The upper layer is in contact with the air above. It is warmer than the lower layers, and so tries to melt and become water.
When we move on ice, we are actually helping it become water. I’ll tell you how. Our shoes (and skates) rub on ice, causing friction. Friction warms the ice further, making it more water than ice. This makes ice slippery.
So it’s actually the water on the ice that makes us fall – not the ice itself.
How does gritting help? Go here to find out.