Why oceans are storehouse of minerals?

I'm not sure what kind of resources you are referring too but it is in every way.

Life

There are a lot of life forms in the oceans, many we don't even know about. Much of it, still untouched by human civilization.

Energy

The ocean is one big power plant, the water currents, the waves, the tides. All this could harnessed as renewable energy.

History

A lot of historical artifacts are laying on the bottom of the ocean. Sunken ships of human history, or fossils dating back millions of years when that part of the Earth was still above water.

Materials

There is a lot on the bottom of the ocean we can not reach. And I'm not talking about sunken ships with a gold treasure. Just like on land you can find gold mines, silver mines, oil and so on. But it's impossible to mine those resources while they are still below the bottom of the sea.