One thing is that the piano was invented in the 18th century. So, music from the early part of the century (like J.S. Bach's music, 1720s and 30s) was originally performed mostly on harpsichord. But by the end of the century (Mozart/Beethoven, 1780s and 90s), the piano was becoming common. That led to the development of the piano concerto and solo piano recitals. Actually in Beethoven's lifetime the piano changed a lot, the range (lowest to highest note) expanded and it produced a much louder sound.