Generally, you need to look at who were the patrons, which were mainly those with money: The Church, wealthy merchants, and the Nobility. While I cannot talk about merchants, and the Church has lost income due to a variety or reasons, Patronage has fallen due to 5th fall of the nobility and a change of economics.

During the 18th century, patronage was well alive and continued as long as there was money to throw around. However, the big paintings and grand symphonic music was being traded for more consumable paintings for the middle class and chamber music that could be played by anyone rather than government funded musicians.

Generally, it's due to the rise of the middle class since taste and wants changed in their favor and away from the nobility.