Why  viruses did not find a place in Whittaker's five kingdom classification?

They are larger than bacteria and can be easily classified under existing kingdoms.

They are cellular organisms that do not have a crystalline structure outside the living cell.

Viruses are non cellular organisms with an inert crystalline structure outside the living cell, not fitting the definition of 'living' based on cellular structure.

Because they are considered to be truly living organisms with a complex cellular structure.