Why is metal hard?

First of all, let me tell you that not all metals are hard under STP (standart temperature and pressure), for example, the well-known metal mercury is liquid under room temperature and one atmosphere of pressure. However, most metals are hard because of metallic bonds. As the atoms of a particular metal draw close together, their respective electrons do not stay attached to the atoms. Instead, they detach and for a "electron sea" in layman's terms.

Individual atoms and free electrons.

As you can see from the atomic structure, a metal is hard because it's atoms are close together, forming layers.  Gasses, on the other hand, have molecules that drift far and wide apart, thus creating a large volume, low density compound. The free electron sea is also the reason why all metals conduct electricity, the electrons are free to move, that is to say, they carry the current.