The simple reason is that if hydrogen crystallizes, there is one electron per site. If there are two electrons per site, spin up and spin down will both be occupied, so the band is filled. This is the case of Helium.. If there is only one electron per site, the band is half filled, and electrons at the fermi level are free to move under an external field. This is a simple explanation in terms of band theory.
There is also the question of whether or not metallic bonding can occur in hydrogen -- here the coulomb interaction is important and you will need to know some many-body theory. You can probably find a treatment in any good textbook on many body physics.
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