Only 5% has been explored because of the extreme conditions found at the great depths, including immense pressure, near-zero visibility, freezing temperature, and lack of sunlight. Which makes it incredibly difficult to send humans or even specialized equipment down to explore the vast areas of the floor. 

The percentage is so little because, it would be very difficult to send anyone or anything down there due to the environment. It also is very dangerous to go down there due to how dark it is because none of us know what's down there so it's very unpredictable of what you could find or see down there.

According to the NOAA (the group that's exploring the ocean) they are continuing to explore the ocean everyday. They are also continuing to discover new feature and creatures. But the ocean will never fully be explored.

Some problems that come with exploring the ocean are the possible diseases. Also how cold it gets while being down there and also the pressure of the water is very dangerous.

So I couldn't find an actual explanation but I personally think they stopped because they discovered something that was frightening enough to make them stop exploring and move to space instead. Also because it's very dangerous.

So there is no proof that there is human life under water. Also it's not possible for a human to live under water due to the pressure of the water and the temperature. The salty taste of the water and there could be diseases.

There is no actual explanation all I can assume is that it just got very dangerous.

I think that there could possibly be lots of unknown creatures. Also unknown plants, sea life critters, and just a bunch of animals or things we never knew about.

The deepest part of the ocean is called the Challenger Deep and is located in the western Pacific Ocean in the southern end of Mariana Trench. According  to the NOAA the Challenger Deep is 10,935 meters (35,876 feet) deep.

So they actually don't send people to the deep ends due to the temperature and how dark it is. But if they were to they would have to go through the intense pressure of the water, the extreme cold temperatures, and the total darkness and corrosive saltwater.