The main feature that distinguishes the piano from the harpsichord is the ability to control dynamics through touch.
Why?
The harpsichord is very similar to the piano in the fact that by the percusion of a keyboard, a set of levers plucks strings to achieve sound.
The main difference is that the piano uses hammers to strike the strings while the harpsichord plucks the strings. That means that the force and dynamics of the striking can be controlled in the piano but it can't be done in the harpsichord.
