Why piano is the main instrument used in the classical period?

Answer 1

Piano wasn't invented until midway through the Romantic period. If you're speaking of keyboard instruments such as the harpsichord, clavichord and organ, you would also be mistaken. The primary instrumentation of the classical period was in the strings. This is why there were so many String Quartets that appeared during this era. Keyboard was not used nearly as much as the strings.

If you listen to recordings, perhaps this is where the confusion comes from as many are played in piano despite originating from different instruments. This is because, piano is the instrument that allows the most depth. Its sheer simplicity to play a single note allows for vast complex polyphonic styles that any other instrument would be incapable of doing. Piano is the most flexible and comprehensive instrument who's nature allows emulation of nearly any work on the planet outside of pure rhythmic pieces.