I'm not the one who came up with the extension. It just sort of organically happened I guess. I'd prefer "zstd" myself, but, frankly, "zst" is fine as well.
Four letter extensions work really well for .java and .json. It seems strange the abbreviate zstd anymore.
I'like to point out though that "zstd" is itself an abbreviation, and "zstandard" would be quite onerous.