Scientists who study forms of marine life that lived more than 200 million years ago usually have to obtain fossils not from the sea floor, but from areas that were once undersea and have been uplifted onto the continents. why

The probable reason for the case would be that the sea floor 200 million years ago is now above the sea level.  

The ocean floor is the eventual recycling center. While the Earth's continental crust can prevail for billions of years, the movement of tectonic plates results in subduction, that is when the crust of the ocean is pushed down into the present mantle. So, the ocean floor hardly endures longer than 200 million years.  

thinking it would be physical characteristics