Shimotsuki Kozaburo made Enma and Wado Ichimonji, which absorb Busoshoku Haki from the user. How did Enma work differently in the case of Zoro and Kozuki Oden?
I assume the following two points are mentioned in some way in the manga.
- If an ordinary soldier uses the sword, his Haki will be drained completely just by a single swing (? I mean, just using it once).
- Oden was the only one who could 'tame' the sword.
So most probably, the answer to your question is that Zoro is still not strong enough to tame the sword completely.
This site stipulates that Zoro's Haki is not completely being drained because either (1) the total amount of Zoro's Haki is much larger than an ordinary soldier or (2) he has control over Emma to some extent.