If we assume that the Brachiosaurus and the modern Giraffe with their long necks both came to be by the need to reach higher-situated vegetable nutrition (which is a somewhat controversial assumption to make, I understand Evolution of long necks in giraffes), what biological reason is there we don't see any kind of modern day equivalent to the Elasmosauri?
Put differently, what environmental factor promoted the evolution of water reptiles with extremely long necks that ceased to exist in a post-cretaceous world? What benefits did Elasmosauri have that later water creatures would not?
Assuming that evolution is shaped by environmental factors, of course.
(And assuming the "water giraffes" weren't simply called "hydras" and extirpated by the ancient greeks. :P )