Why is the "Defender of the Faith" part of the Queen's title only used in some countries?

For example, in Canada her full title is Elizabeth the Second, by the Grace of God, of the United Kingdom, Canada and Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith, whereas in Grenada it's Elizabeth the Second, by the Grace of God, Queen of the United Kingdom of Great Britain and Northern Ireland and of Grenada and Her other Realms and Territories, Head of the Commonwealth.