WHY do most standard pianos have 88 keys and WHEN and HOW was that decision made?

The de facto standardization of the piano keyboard occurred in the 1880s and 1890s. There was no driving force behind it - it was just something that happened.

https://en.wikipedia.org/wiki/Innovations_in_the_piano#Extended_keyboards

Even today, however, pianos with a few extra keys (usually in the bass range) are being made.

One possible explanation (mine - not official) is that this specific range does provide for seven full octaves in both C major and A minor - the two scales that do not need black keys.

I would assume the limitation isn't the number of keys but the length of the keyboard for somebody to play it. Although there is a bench to slide back-and-forth, you'd still be tethered to its foot pedals.