Because they lack many of the properties that scientists associate with living organisms. Primarily, they lack the ability to reproduce without the aid of a host cell, and don't use the typical cell- division approach to replication.
a virus is considered non-living because of multiple reasons but primarily because they cannot reproduce on their own. other reasons would be because they cannot create energy (or really anything else for that matter) without a host cell< another is the only carry one type of nucleic acid where as cells have both DNA & RNA.
