It's a misnomer. The general American public weren't aware that baseball had taken off in other countries, like Japan and Cuba. Originally a Championship Series, the powers that be were probably influenced by someone that hinted World Series had a better ring to it.
So, even though it's called the World Series, it's really the USA-Canada Series. Hopefully, and this would be reliant on travel technology advances, it will be a true World Series someday.