Why does molecular hydrogen react slowly at room temperature?

Molecular hydrogen ($H_2$) reacts slowly at room temperature primarily because of its very high dissociation energy.

To participate in many chemical reactions, the bond holding the two hydrogen atoms together must first be broken, a process that requires 104 kcal/mole of energy. At room temperature, there is insufficient energy available to overcome this high activation energy, rendering the molecule seemingly inert.12

Because of this stability, molecular hydrogen typically requires elevated temperatures, sparks, radiation, or the presence of a catalyst to facilitate reactions at a measurable rate. When these conditions are met, the molecule can dissociate into highly reactive atomic hydrogen, which readily combines with other elements and compounds.13