Use of the Eisenstein Criterion

Question

I am looking at the Eisenstein criterion and this works for unique factorization domain. Now I am asking myself if $\Bbb Q$ is a unique factorization domain? I would yes because it is a field. Now why we use often the criterion to $\Bbb Z$ and conclude with the Gauss lemma that it is irreducible over $\Bbb Q$ instead using it directly to $\Bbb Q$?