Why is there no more Voynichese in any other manuscript?

[Jorge_Stolfi]

we know that the VMS was used heavily, due to paper fading, so that means that many people used it for practical purposes and were able to read it without many problems.

The first part is true: the original ink has faded considerably (which is more visible in the drawings than in the text, for reasons that  won't go into here).  Although that was probably due to the type of ink being unsuitable for use on vellum.

But the conclusion does not follow.  Most of the "heavy use" happened in the ~600 years after the book was created.  Baresch, for one, seems to have been obsessed with the VMS for much of his life (in the mid 1600s), and surely leafed through it many, many times.  And I bet that, between its donation to Beinecke and them deciding to lock it away, it was the most consulted manuscript in their possession.

There is no evidence at all that anyone else besides the Author could have read the VMS, or were supposed to.  In fact, there is (admittedly very thin) evidence suggesting that the Author himself could not read it -- because he was effectively blind by the time he decided to have the book copied onto vellum. This evidence is the many apparent errors made by the Scribe who did the copying, which were not corrected or amended.  

All the best, --stolfi