It is so because metals cannot be extracted from minerals in a convenient way. E.g.:- Aluminum metal is found in both minerals: clay and bauxite. Aluminum metal is extracted only from bauxite in a convenient way. So, bauxite is an ore of aluminum. On the other hand, it is not extracted from clay so clay remains only as mineral not ore of aluminum.
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To be called an ore a mineral must be a  raw material which is used for the production of a particular metal or other product. For example Galena is an ore of the metal lead and magnetite is a major ore of iron. On the other hand calcite is a mineral of carbonate that that is not used for the production of metal and so is not regarded as an ore. 
That's why all ores are minerals but not all minerals are ores.