Genetically, many more humans are right handed than left handed. When everything needs to be done the same way, therefore, there is a natural tendency to require that the procedure favor the right handed.
For example the Roman army required all foot soldiers to hold the shield with the same hand, for the sake of tight formations such as the Tortoise or the Shield.
Formal Roman meals were taken reclining on one elbow. You reclined to the left, and ate with the right hand. Jews also celebrated feasts including Passover this way (as in the Last Supper, despite later European depictions). In Judaism left handedness is accepted, but only as an unfortunate imperfection along with blindness, lameness or a lisp. Any of those imperfections disqualified a priest from serving in the Temple in Jerusalem.
In the Arab world today there is still a heavy emphasis on right handedness. People often pick food from a common dish at mealtimes, and the right hand is the “clean” hand for shaking hands, for writing, and for eating with. The left hand is used for wiping yourself at the toilet. Imagine the personal embarrassment and the social stigma of the thief who has had a hand cut off, and must do everything with the same hand, and dip into the common pot with his “dirty” hand…
The left hand ends up with all kinds of negative connotations, now built directly into many languages. And so the final word has to go to the trilingual quip by Canadian socialist Tommy Douglas: “The left in Canada is more gauche than sinister.”