What is one reason people used to think that nothing could live deep down in the ocean

I can answer it. The answer is that we had not explored the ocean yet and we were not able to do so for lack of better technology.

Because of what people experienced themselves. the public thought that no living thing could actually live so deep in the ocean waters because 1, we lacked the proper tech and equipment, and the water pressure would crush these submarines and etc.. therefor, people thought, "Hey, since a giant metal thing-a-mabob cant get to the center of the earth through the Atlantic ocean, than nothing can!" and 2, not many people were very open-minded about these theories. causing the incorrect inference, that nothing can live at the bottom of the ocean.