I expect it is because in ye olde days, everyone had to be right-handed. Sword scabbards would be clashed if they were on the ‘wrong’ side, infantry with pikes would not be uniform with ‘lefties amongst the line, archers would clash when drawing their bows or extracting arrows from the quivers. Jousting might have been a bit awkward, too!
All machines were made for ‘righties’, not ‘lefties’. Just like left handed scissors only being available in relatively recent years.
Every one of my circular sock knitting machines is right handed. Only recently manufactured machines are now being offered in either left or right versions.
Go back only a relatively few decades and school children were not allowed to be lefties – they had to conform to the standard of being right handed – no wonder some children’s education was a bit lacking!
’Nuff said? Try picking up heavy workpieces and fitting them in the chuck with your non-dominant hand.