Not really developed for Mac or PC, but any computer that could get a soundcard installed. I think even the name "Soundfont" is really misleading. It's basically just like a Megalith file that contains all the samples, key mappings, and other settings in a single file. The idea was to be able to load the file from disk into the memory of a soundcard and have superior MIDI playback. They were the "go to" audio sample type file when used with MIDI players that wanted better quality then what the System sounds provided (like built in Quicktime).
They were more of a "Pro Consumer" tool since the Big DAW samplers used other formats. So Soundfonts were cool and convenient for MIDI playback, but were limited in other areas. Since they contain a simple file format with PCM data (like a WAV file) with layers, mappings, and other settings they were not meant to be included as copy protected libraries and is probably one reason they were not widely used.
Some samplers load soundfont format (like EXS 24) and they immediately convert, without telling you, into their format.
The last version of soundfonts even supported 24 bit, but the number of layered samples was always kept low (remember they were designed load into a a sound cards memory), so although some soundfonts sound "OK" , they pale in comparison to a HALion or Kompakt patches.