Why is the plural not "soutien-gorges" ?
French is not consistent with regard to the plurals of hyphenated nouns.
The only way to answer that seems to be "Because it's not.".
Presumably because the item being supported (bosom/gorge) is singular - which is a bit odd in itself..!
I just hope we are not quoting a middle-aged man here!
Well... Father Ted, maybe?