The regnal numbers of the monarchy of the United Kingdom are combined from the regnal numbers of the separate kingdoms of England and Scotland. Hence, as there was a previous Queen Elizabeth in England, the current queen is known as Elizabeth II of the United Kingdom of Great Britain and Northern Ireland. It is a common mistake to call her the Queen of England: she is not, as England is no longer a kingdom in its own right, but instead forms part of the singular United Kingdom.
However, Canada, Australia, and Jamaica, among others, are kingdoms in their own right, with their own monarchies (which merely happen to be vested in the same person). Elizabeth is not Queen of England, but she is Queen of Canada, Queen of Australia, and queen of many other countries.
In these countries, why is she known as Elizabeth II? Elizabeth I of England never ruled any of these countries.