The simple answer is "because there is no need to". In the past the congress had much less power, limited to only those powers given by the constitution. The rest was up to the states.
This is still the case, but since the 1930s and 1940s the power of the federal government expanded greatly after a series of Supreme Court rulings, mostly related to the Interstate Commerce clause in the constitution which has since been widely used (and according to some, abused) to pass a wide variety of laws that would have been impossible before.
This is why a constitutional amendment was needed to prohibit alcohol in 1919. See Why was the prohibition of intoxicating liquor enacted through a constitutional amendment? for more information.
Besides, all the truly "important bits" such as universal suffrage or Presidential succession have been done already. Some might argue that the U.S. political system still needs a lot of changing/fixing, which is why amendments are still being proposed all the time, but few are as acute as, say, allowing women to vote.
In addition, current levels of polarisation make it difficult to actually pass anything.