Ah, found it.Think this is the reason but to be fair tachographs have shown KM since their introduction decades back, where the stipulation for KM comes from I couldn’t say tbh.
https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1985R3821:20111001:EN:PDF
I am surprised that you've never been curious enough to read the legislation given its your employment! But yeah, EEC rules since 1985.