Because the mounting peg was on the left end of the rear axle on early safety bikes? You put your left foot on the peg and lifted yourself forward and snagged the right pedal with your right foot and pedaled with one leg for a moment and then lifted yourself into the saddle and pedaled normally - the whole procedure being a carry-over from riding ordinaries/penny-farthing bikes. I don't recall ever seeing an ordinary with the mounting step on the right ...
Russ Fitzgerald, always fascinated by the dead hand (or pedaling foot) of history, in Greenwood, SC, USA