An estimated 13% of the population is left-handed. Sam Colt was, along withbeing a brilliant firearms' designer, a smart business man. Do you really think that he would, after struggling through a failed attempt with the Paterson company, alienate potentially 87% of his customer base by catering his sixguns to the left handed?
And again, as I noted below, if the 1873 Single Action Army was made to order for the military, do you really think they would've ordered it differently if the loading gate was on what they perceived to be the wrong side?
Of course not, on both counts.
And so why is every other sixgun built the same way then? Remington's percussion and cartridge conversion sixguns had their capping notch and loading gates on the right side. Their cartridge conversions preceded the Colt by a couple years, due to their licensing of the Rollin White patent.
It's also worthy of note that no other firearm to come from Colt was designed for lefties. Their rifles all had their capping notches and loading ports on the right side, to facilitate loading and capping with the more dextrous right hand.