Because of Ludwig van Beethoven. His work was the turning point in the development of symphonic music. Starting out as a classicist composer, which is especially visible in his first two symphonies, Beethoven gradually turned to a broader understanding of the orchestra's dramatic power. He started composing longer symphonies for larger orchestras. The climax was his 9th Symphony, where he also employed a large choir and vocal soloists, thus triumphantly signifying that the age of classic, small-scale symphonies with firm structure, has ended. The composers before him used to write a few dozen, even hundreds of symphonies and concertos. The composers after him (including himself) would usually compose up to 10 symphonies and a couple of dozen concertos for solo instruments and orchestras.
