The extinction of the dinosaurs from the impact of an asteroid – Chicxulub, in Mexico around 66 million years ago, in addition to resulting volcanic eruptions in the Deccan traps in India, and in other places destroyed most species, not just dinosaurs.
The key process at work appears to be the resulting eradication of most plants dependent on photosynthesis, as solar output received at Earths surface became severely reduced from particulates present in the atmosphere.
With the loss of their food source, massive numbers of animals and insects went extinct, as did those that prayed upon such animals that ate those plants- in total, close to 70%.
In the oceans, fish, mollusks, sharks, and many varieties of plankton were also severely depleted although many survived in enough numbers to allow their survival. Scientists believe that the most successful surviving animals were carrion eaters and those that fed on carrion eaters.
Among marine animals, bottom feeders -living on detritus- survived better than animals or fish that lived near or close to the surface.
To answer your question, it appears that because Plesiosaurs breathed air, they evolved as predators eating surface and near surface fish and simply failed to survive as their food supply diminished below sustainable numbers.
Among reptiles today, only sea turtles, marine iguanas, some crocodiles and sea snakes still live in the oceans. Both ancient sea turtles and crocodiles could eat carrion. That may explain why they survived extinction. More recently Marine Iguanas are thought to have left land about 8 million years ago while sea snakes evolved on land and are thought to have moved into the oceans about 10 million years ago, both well after the K-T extinction event.