Because there was only one key on the flute you had to play with very complicated fingerings. These fingerings are called fork fingerings and the tones of these fork fingerings sounded not too good. There came extra holes and keys on the flute to get rid of the fork fingerings. Around 1775 the flute had four keys: d sharp (already existed), b flat key, g sharp key and f key.
Eventually more and more keys were added to the flute, but there still wasn't a good system for the keys. The very famous English flautist Charles Nicholson (1795-1837) introduced larger holes for the fingers and a larger hole for the mouth piece in order to get a better sound.