Richard Wagner played a big role in making orchestra popular. Wagner was an opera and orchestra composer. He made orchestra bigger and popular by introducing many musical instruments aside the typical orchestra instruments that were mostly string instruments in the 17th century.
He introduced these instruments mainly for his compositions. This made orchestra bigger and theater halls larger. Other composers started composing music piece for variety of sounds. Before Wagner introduced extra instruments, orchestras were small and were limited to certain instruments.
Mozart, Beethoven and Mahler are also composers that made orchestras popular.