Here's why:

* Viruses are not cells. They are much simpler structures than cells and lack many of the components that define a cell, including a nucleus.

* Nucleus contains DNA. Viruses can contain either DNA or RNA as their genetic material, but it's not enclosed within a nucleus.

* Viruses lack organelles. Organelles, including the nucleus, are structures within cells that perform specific functions. Viruses don't have these internal structures.

Instead of a nucleus, viruses have a protein coat (capsid) that surrounds their genetic material. This capsid provides protection for the virus and allows it to attach to and enter host cells.