The driver’s side varies by country based on which side of the road you’re driving on, because if you drive in the middle of the road you have a wider range of sight, especially for oncoming vehicles. So for example, in Britain cars are made with the driver’s seat on the right side because they drive on the left and that puts the driver closer to the middle of the road. So, I think the question you want answered might be just as much why primarily just Britain and some of their former colonies drive on the left side of the road whereas most of the world drives on the right side. I don’t know the answer offhand, but I’m pretty sure I’ve seen that posed here before, so once I’m off mobile I’ll try to find a link to that.

Edit: Okay, this post by /u/doc_frankenfurter provides a few different european examples and a citation for further reading. Though the ancient greeks and romans apparently had guidelines for left-side traffic, there isn't enough clear continuity to determine the rationale for every nation's initial choice to use left hand traffic or right hand traffic. However, if you look specifically at Europe today, the continental countries now all use Right hand traffic, largely just because the custom reached critical mass and it didn't make sense for any single country to drive on the opposite side when compared to their neighboring nations. The countries that use left hand traffic are all island nations without that constraint or the same motivation to follow suit, notably Britain and Ireland. British Imperial rule just prior to and during the period that auto-routes were being developed did lead to many of their former colonies adapting the British custom of driving on the left.

Again, for practical and safety reasons you want to have the driver on the side closest to the mid-point of a road, so the choice of right hand or left hand traffic, however arbitrary, then influenced the side drivers sit on in different nations.