It's mostly because of Zoro haters and Luffy wankers. They can't stand for Zoro to be powerful. Especially since his power comes from himself not outside power like magical fruit.
DF has played a major role in the manga and been a major power since the beginning. We know some characters are powerful only because they ate a magical fruit. Since that wasn't how Zoro got his powers they didn't care.
After Oda introduced enma some saw it as any opportunity to try to downplay Zoro. Even though the manga told us all enma does is try to force the user to release their haki if they can control it. Of course, they pushed their headcannon by claiming it did more like give Zoro's other user haki, CoC, and more.