Why does the iPhone 4 lifeproof case only go up to 6.6 feet underwater?

I'm going to buy one of these case but I'm just curious to what happens if you go further than 6.6 feet?

The deeper you go, the higher the water pressure gets. If you get below 6.6 feet, the water will start to force itself into whatever seals keep the case waterproof.

It's probably the amount of pressure of water on the phone. If you go further then 6.6 feet then the pressure will cause the water to leak into the phone.
A guess

Probably nothing.

This is more than likely a safety margin. What it means is it's guaranteed up to 6.6 ft.

The reason depth matters is water pressure. Since the case is not a solid covering (meaning you can open it, plug in headphones, etc… ) waterproofing is done using seals. Basically gasket material is pressed together tight enough to form a waterproof barrier. As water pressure increases with depth, the water is pushing harder and harder to get in. Go too deep, and the seals fail.