This sounds to me like your doing a high school report or something and want answers...he heh.Anyways, The piano became more popular in the 18th century and especially the 19th century because there was a shift from sacred vocal music to secular instrumental music starting about 1600 and going until early 1800's. Remember the idea of composing music for "arts" sake is a very new concept that really didnt develop until Beethoven and finally fully matured with the romantic period. (Roughly 1827 to 1900.) Vocal music encompasses a very limited range as well as many of the orchestral instruments at the time. Having 8 octaves on a piano was deemed unnecessary in say -Montiverdi's time. The 7 octave piano wasnt developed until the 1820's. Partly because the cast iron frame to support such tension wasnt developed until after the industrial revolution. As orchestration techniques developed and orchestras got larger, composers and pianists wanted a bigger louder instrument to compete with the orchestras. Of course, composers use the piano to compose their works themselves and want to imitate the instruments they are writing for (orchestral reduction) but more importantly the piano needed to compete with the huge sound of the orchestra in the romantic concerto. An Iron frame was needed for volume etc.Hope this helps...Im sure you can google piano or something and find better info.