why do some animals live in the low tide zone and others in the high?

Most sessile inhabitants of the upper shore CAN exist on the lower shore, but are outcompeted by other species there. Those which are more resistant to dessication, but not highly effective competitors in their ideal environment, are thus often restricted to this zone. Some animals, particularly some crabs, cannot survive being covered with water for long, and as such must live on the upper shore or have watertight burrows.

Basically, they live where they can, not where they want to, so animals that can't survive being baked in their shells every day, or high salinity and low oxygen if they live in rock-pools, don't. animals that can, do, and that pretty much sums it up.

that was quite repetitive, wasn't it?

I've done my best to forget the Shannon diversity index completely, so I can't help you there, sorry.