Mainly because of tradition.
A possible "reason" is that it's because of the English influence. If you ride on the left side of the road, then you'll want to mount the bike from its left side. So if you stick the chain on the right side there's slightly less risk of rubbing up against the oily chain.
For a country with right side traffic you should as a rule overtake a vehicle on its left side. But on a bicycle it's allowed to overtake on the right side as well.
Answer is "Chain"
Its all to do with the design of the gearbox and mounting of the back brake. Royal Enfield still chain on the right and Harley Davidson run belts on either side.
Keep all the parts of the drivetrain properly cleaned, lubed and adjusted.
There are things called arm cycles, which basically are wheel chairs with a bicycle drivetrain added to them, with the cranks being turned by the arms.
Left and right on a bicycle is given according to what would be left and right for a rider, facing the direction of travel. To avoid this confusion, some people use drive-side (DS) and non-drive side(NDS) instead. Drive side being the side with the chain on.
you get to the other side
The one that fits you the best. There's a small advantage to approach a bicycle from the left side. The right side has the chain, and the chainrings. These are usually a bit dirty, so you tend to get stains on your clothes/legs if you touch them.
A bicycle for two, one after the other, is usually a tandem. A bicycle for two side by side(very rare) is called a sociable. If it's about a kind of race, then that race is called a pursuit.
It is legal to ride a bicycle on a right shoulder of an expressway.
If there is no bike lane, then the appropriate place to ride is on the roadway. As a slower vehicle you must abide by the rules of the roadway and allow any traffic you are holding up to pass.
On the east side of GoldenRod City in a little house.