Why isn't hydrogen considered an alkali metal ?

Hydrogen isn't classified as an alkali metal because it exhibits several properties that are not found in the alkali metal group, including the tendency to be found as a diatomic molecule, a tendency to form weak bonds with other bound hydrogen atoms nearby, weak electromagnetism, a low activity series placement, and the tendency to be found as a gas. Because of all this, it's impossible to classify it with any group, although it does seem to be closest to nonmetals.

Hydrogen is a non-metal, it is one of the two elements on the periodic table that only need two valence electrons to be stable. Hydrogen doesn't share many properties with the other Alkali Metals. It has a much higher electronegativity, doesn't form ionic bonds, doesn't react with the same things that the other elements in group one do (like water.) The only reason hydrogen is place there is because it has one valence electrons like all the other alkali metals and there is not where else to place hydrogen, because it doesn't really fit in with any group.