Why would the absence of a particular trait not exclude a species from a particular clade that possesses that trait?

The absence of a particular trait does not exclude a species from a particular clade that possesses that trait because physical attributes of the species is not the basis of the groupings in the clade.

Cladistic groupings uses the evolutionary relationships among the species as its basis in grouping rather than its shared physical traits. These evolutionary relationships are called apomorphies or a set of unique features shared by all organisms in the group but not found in the distant ancestors.