Because it is not. If you ask "Why is Manu Smriti not a Tantra or not a Purana?" what will be the answers?
Dharmashastras are the Smritis, like Manu Smriti etc..
Srutis tu vedo vijneyo dharma-shAstram tu vai smriti ||
Manu Smriti 2.10. But by Sruti (revelation) is meant the Veda, and by Smriti (tradition) the Institutes of the sacred law: ...