Because it does not lower your E2 per say, it for the prevention of gynocomastia.
From the Nolvoadex profile page.
Nolvadex Side Effects
Unfortunately, Nolvadex isn’t perfect. Anecdotally, it has been linked to reduced gains in some TrueMaxders. This isn’t due, as previously thought, to its reducing estrogen levels (which it doesn’t), but rather to its ability to possibly reduce IGF levels, which are important for muscle growth.