Piano is often just short for Forte-Piano, the instrument's christened name. Why is it that the more common name did not become the Forte?
Is that just a thing that fate decided or was there someone who decided on the piano?
Piano is actually short for pianoforte, not fortepiano. The fortepiano was an earlier version of the piano, used in the times of Haydn and Mozart, that had less sustain. The modern pianoforte was not invented until around 1800. It seems the pianoforte was shortened to piano simply because it was the first few letters of the name (the first few letters are the part of the word you hear first and so it makes sense to abbreviate like this) and also, as Super Query Lad pointed out, "forte" was already a word in English while "piano" wasn't.
"From the late 18th century, the fortepiano underwent extensive technological development and thus evolved into the modern piano."
"In the period from about 1790 to 1860, the Mozart-era piano underwent tremendous changes that led to the modern form of the instrument."
And the name of the very first instrument was actually "gravicembalo col piano e forte" (roughly “soft and loud keyboard instrument”). In 1709, it was first revealed as the invention of an Italian harpsichord maker named Bartolomeo Cristofori.Article
"The word “fortepiano” is sometimes used to describe the pianos of the late 18th and early 19th centuries. In some languages, such as Russian, “fortepiano” is the normal word for a piano." Article
Not a lot of info on the hows and whys of it, otherwise. To speculate: 'Piano' is the shortest, easiest way to describe the instrument in the English language - and we English speakers love to chop down words into tiny little bits. Also, the word 'forte' is somewhat of a synonym for an individual's strengths. "His forte is playing the piano," for example. Whereas, the word 'piano' doesn't really have any other connotations (at least in the English language,) so it makes since that it's used to refer to the instrument.
(This is a language or history question, for sure. [And, yours truly added the 'history' tag, FYI. Just don't have the creds so, it had to be peer reviewed before it was added.])