It's often arbitrary, developing from historical regulations/conventions for pre-automotive transport (horses, carts, etc). And yes, a few countries have made the switch, mostly notably Sweden in 1967.

Here's previous post about the subject found in the FAQ:

https://www.reddit.com/r/AskHistorians/comments/1com7k/why_does_north_america_drive_on_the_right_side_of/