Why are people from Canada called Canadian but people from America are called American?

It's origins are political. Canadian was first mostly used by French speaking people in what was New France (Quebec) to distinguish themselves from the English who by that point had largely chased the French out of what is now Canada.

Then the war of 1812 happened which in Canada too this day is seen as an American invasion of Canada and an attempt to annex us.

After the war English speakers here started calling themselves Canadians because they didn't want to be associated with United States.

Before the war there were many English people in Canada that were sympathetic to the American independence movement but the war of 1812 burned that good will forever.