Why aren't all metals included in the activity series ? For example why isn't molybdenum included ? Now we know that hydrogen displaces those metals from their oxides that are less reactive than iron. But how do I know whether Hydrogen would also displace Mo if its not there in the reactivity series ?
That said, as @Karl mentions, reactivity series I've seen usually are incomplete, often leaving out transition metals with multiple valences such as Mo and W. To fill in the missing species, look at a good table of the electromotive series.
The omission of so many metals is downright shocking, revolting. One would think a current table would have included more metals; these reactivity series seem like works in transition.