The decline of federalism has much to do with the 13th/14th/15th amendments and the Civil War. Between the Civil War and the Civil Rights Era, it became apparent the states could not be trusted. Thus the 14th amendment gives the federal government explicit power to legislate against the states in certain circumstsances, and incorporation of the Bill of Rights through the 14th amendment has dramatically limited state power in other areas.
Most of the rest of the change in the balance of power has come from the changing nature of the economy. Both the plain text and the early interpretations of the Commerce Power (Gibbons v. Ogden in 1824, a mere 35 years after the ratification of the Constitution) support the modern exercise of federal power. What is different is that in 1789, most people lived on farms, grew their own food, made their own clothes, etc. Very little human activity came within the scope of the commerce power, as broad as that power was defined. Today, just this post involves an interstate electronic transaction. It's nearly impossible to buy clothes that weren't made in a foreign country. Etc. Nearly all activity comes within the scope of federal power because nearly all activity is commercial and either interstate or international in nature.
Most of the rest of the change in the balance of power has come from the changing nature of the economy. Both the plain text and the early interpretations of the Commerce Power (Gibbons v. Ogden in 1824, a mere 35 years after the ratification of the Constitution) support the modern exercise of federal power. What is different is that in 1789, most people lived on farms, grew their own food, made their own clothes, etc. Very little human activity came within the scope of the commerce power, as broad as that power was defined. Today, just this post involves an interstate electronic transaction. It's nearly impossible to buy clothes that weren't made in a foreign country. Etc. Nearly all activity comes within the scope of federal power because nearly all activity is commercial and either interstate or international in nature.