Pianos

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Because it's formal and proficient. I'm not exactly sure how someone thought it up, but the modern day piano has only existed since about the late 1700s. They used organs, harpsichords, fortepianos, and other keyboards before that.

>>3280292 (OP)
A piano allows one person to play multiple separate melodic lines simultaneously and to play all sorts of complex chords, while having much better dynamics (ability to shift between quiet and loud) and control over intonation than a harpsichord and being much cheaper and more portable than an organ. Composers loved them because you can effectively compose for a whole orchestra on one, and you can also improvise very effectively while making a lot of musical choices on one. Regular people loved them because back then, before recorded sound, one of the most common ways to listen to music would be to play it yourself from a score (or have a friend do it), and pianos let middle class people do just that. On a piano you can play a simplified version of an entire orchestral work that gets the main thrust of it across. (Or even more, if you're a virtuoso - for example, see Liszt's piano versions of Beethoven's symphonies).
As for the technical details of how it developed, I don't know much about that, but it's an extensively researched subject, so you should be able to find what you're looking for.