Why does Canada have a federal government?

Answer 1

It's complicated but the many provinces and colonies in Canada would not have joined into a Confederation if Britain had been willing to provide protection from the United States.

Fear of American expansion was very real, particularly after official and unofficial invasion attempts in 1812 and 1813, and 1866 combined with decreasing interest from Britain to protect it's North American Assets lead to a Confederation of 4 provinces in 1867.

It was that Confederation, created in response to the territorial desires of their southern neighbour, which created the need for a Federal Government.