One factor:  

IT’S THE ECONOMY, SNOOPY

Another factor is cost. Larger dogs are typically more expensive to feed and care for, so when the economy is bad, many people gravitate towards smaller dogs, according to the AKC. After the recession in the ‘70s, both the GREAT DANE and the SAINT BERNARDbegan a long slide.

Great Danes, however, have made a comeback recently, weathering the ‘01 and ‘08 financial crises, while Saint Bernards continue losing ground. “We saw big breeds begin to rise in popularity starting back in 2013, potentially the result of an improving economy,” says the AKC’s Vice President Gina DiNardo.

I did not realize they are more expensive. 

Big breeds in general do not live as long so maybe the lifetime cost is about the same.