Reimu does a lot of things that one would describe as 'shooting herself in the foot'.
She locks the majority of her karaoke streams behind her Membership, meaning people rarely experience her main talent. She usually streams during the most packed slots and has to contend with Pomu and Selen, and she also doesn't play — if you'll excuse my /vt/ vernacular — 'buff games' very often.
It's not that NijiEN fans aren't interested in her because she IS inclining, it's just that she hasn't really tried to stand out. Japanese fans weren't even aware she was fluent.
Of course, I imagine that if she cared about any of these things, she'd already have changed them. She doesn't need Vox to gain momentum. We are only a couple months removed from Enna and Rosemi struggling to break 1K.