The Voynich Manuscript is a cipher because...?

[Emma May Smith]

It can be difficult to separate a cipher from a constructed language.

Maybe a better question would be, why is the Voynich Manuscript not a natural language?

It's really not better to phrase it that way. I want positive reasons for the Voynich manuscript being a cipher. If folks' beliefs truly boil down to, "it's a cipher because I don't think it's a language", then I think the whole side of cipher research on the VM has a serious problem.