The history of the harp knows a few crucial moments where it could have gone different ways with the development of the instrument. It is like so many things in this world that different factors can be part of something surviving or not, you can surely say this about flora and fauna, technology, politics etc.
Due to the musical climate changing a lot towards more complex structures in the 19th century. The harp, which was originally an instrument that could play limited numbers of scales (changing pitches by hand), was falling behind and became less and less popular amongst composers to write for it. It happened that two French harp makers at the beginning of the 20th century were convicted they could make the harp of the future: Erard and Pleyel. Erard came up with the double-action pedal harp, a harp with seven pedals to raise or lower the pitches, using the feet. Pleyel, on the other hand, believed in the design of the piano and make a harp with cross-strung strings, one row for the white notes and one for the black notes. This meant you had two diagonal rows of strings, crossing each other half way. Explaining this concept already makes me dizzy and that was pretty much also the case with playing this harp. A complex playing technique was invented to play this harp. To promote it to the people, Pleyel asked Debussy to write a piece for this specific harp, which became the Danses for harp and string orchestra. And during that time, on the other side of the city of Paris, Erard asked Ravel to write a piece for his pedal harp, naming it Introduction et Allegro.
To make a long story short, both harps were played by harpists but after a while it was clear that the pedal harp gave more possibilities and a bigger volume. The cross-strung harp fell into oblivion and Debussy decided to rearrange his piece to the pedal harp, thankfully!
(Pleyel’s cross-strung harp)