What are the main reasons that cause most of the water on Earth to be unusable?

Like most materials found on Earth very little of it is already in a usable state.

Basically, there is a lot of water, but as we get more and more people the “ready to use” water is quickly spoken for.

There are still huge amounts of water left. Only now it requires prep-work before its “ready-to-use”.

But, since its been so easy to get “ready-to-use” water for so long we don’t have many places that can cleanup whatever has been mixed into the water (dirt, pollution, salt … whatever).

So, the simple answer is straight forward simple contamination coupled with higher demand.