I can get both green and black olives in jars or at places like Whole Foods at the appetizer bar.
Not of fan of tinned olives since they taste well tinny, LOL.
I'm assuming you mean the cheap little black ones that come in a can at the supermarket? The black olives come in a can due to botulism outbreaks in 1919 and 1924 when they were marketed in a jar. Nobody would touch em so they switched to cans.
Green olives are brine-cured at a salinity high enough to prevent bacteria growth, so they can be packaged in jars. Black olives are not, so after packaging they have to be heated to temps high enough to kill bacteria and glass jars would break in that heat, so cans are used instead.
Green olives are picked when they are under-ripe - that's why they are green. Their flavor comes from the pickling spices and pimento peppers and whatever else is in the brine with them for many months. You don't have to pasteurize when you're pickling. Brine is corrosive to cans, though, so that's why they are in jars.
Black olives are ripe olives, so all they need to do is pick, process, put into cans and pasteurize them. They don't need brine (because of the pasteurization) so they can go right into a can.
Green olives are harvested before they are ripe. They are packed in brine-filled jars and cured in the brine for several months. Also, brine corrodes metal cans.