it's the DAW programmers (or marketing, who knows 🙂 who choose the plug-in format.
PTLE uses RTAS, DP uses MAS (but opened up to use others), and Steinberg invented VST and that format really caught on. then apple made AU, and vendors are adopting it at varying paces.
when programming a plug-in to a format, programmers still have to contend with the OS, so you'll see VST plugs that work only on windows, for example. or AU plugs that work w/ ppc but not intel.